I am dealing with the same problem...
To make a very long story short, I have concluded that for me it is likely endometriosis (History dating back to 1993,UC diagnosed 1995, always knew there was a connection)
Three years ago I began to have bleeding ONLY at the start of my cycle which usually tells a smart GI to investigate for endo...He did not wish to, but instead my condition worsened with every drug they added and I ended up hospitalized with liver and pancreas trouble...He finally agreed to do a sig at the beginning of my cycle and I told him he needed to LOOK for endo and forget what his opinion was and to leave his ego at the door...He was more than rough during the sig and made a comment to the nurse that many woman with Crohn's are complaining about exaserbations during their cycle...My opinion is that some is linked to diet, woman tend to increase their dairy and chocolate intake that time of month and that is certainly a known cause esp. with those with delayed food allergies, like myself...
I then ended up hospitalized with a systemic infection from the sig and almost died form it along with the added abx they treated it with...Levaquin destroyed my body while I was fighting the infection...
My biopsies always show inflammation cause my doc always writes "I favor Crohn's" when he sends the biopsies off for pathology but the pathologist never concurs...
Funny, you brought this up today...I began my cycle Tuesday and it is frightening allready to have both at the same time...It is now like vaginal blood coming from the rectum...My new proctologist realized right away that endo could be the cause and wants to do a scope on the exact day it starts but I have been researching this long and came up with my own things I wished to try...
I am having my hormones tested, finally, only waited 8 years (my docs wouldn't order them, they didn't think there was a connection) Also will be tested for copper and prostaglandins, all 3 can be associated with rectal bleeding and copper load can be the cause of UC, which may be in my case...The cycle then increases the estrogen which in turn increases the copper to toxic levels and will bring about the initial bleed, which in my case is the rectum...High levels of copper increase prostaglandins which are at their highest during the start of the cycle...Have I lost you yet? It makes such sense yet my old docs knew none of this nor cared to...I found my 3 new docs and I just keep asking for tests and am coming to my own conclusion...I will avoid future invasive tests until I know what they should be looking for...
Now I am in the process of finding the best surgeon that deals with bowel endo, one in Maine (where I am) and one in PA...
Btw, copper levels can increase when eating milk, nuts, chocolate, beer, water...
Things that make you go HMMMMMMM....I no longer trust the medical profession as a whole and since I have been able to do continued research and testing as I see fit I now only suffer from rectal bleeding and have likely eradicated all the other issues such as arthritis, fibromyalgia, chronic fatigue, raynauds, etc...all known auto-immune diseases are now gone just with dietary changes...Now if I can conquer this rectal bleeding issue once and for all...I am no longer falling for the diagnosis of UC, Crohn's or the more recent Crohn's colitis...I will get to the bottom of it 
Hope you do too!
Can't wait to see what others have to say but as far as I can tell when I google this topic, there is not much to learn from, you basically have to dig around and turn into Sherlock Holmes to find something that 'clicks'...
I just saw you had seasonal and milk allergies, should raise a red flag for you also...This is likely a histmaine reaction but from what??? I am nearly outta my mind trying to figure this out for myself...I did begin LDN two weeks ago, it is known to eliminate endo as well as Crohn's inflammation...Hope it's my magic pill...If nothing else it is boosting my immune system and alleviating all the other things I listed above...Hope you find the answers for you...
Wish you well~MamaDove