Matt,
Welcome to the forum.
It's interesting that you're able to pin your flare or diagnosis with not getting enough vit. d, there is an strong correlation in the scientific studies regarding low vit. d3 and IBD, but many of us haven't been able to personally make that connection. So kudos to you!
As to your other question about a flare, for most of us it is a combination of any of these things:
- increased BM's
- increased urgency with or without cramping
- increased mucous
- bleeding
- changes in BMs, in terms of consistency - whether it be constipation or for most of us the other direction - diarrhea
- increased bloating and gas
- pain and inflammation in the intestinal area or even the left area under the ribs
As to what can be done?
That depends upon what you are currently doing: as to meds, diet, supplements?
There are things IMO that you can control, which will have a direct impact on your disease, the symptoms you experience and their severity.
I was able to control my UC into remission through various supplements, probiotics, diet modification, and LDN , low dose naltrexone.
For more info on the protocol which has put me into remission, please go to this link: https://www.healingwell.com/community/default.aspx?f=38&m=1666318 , and look at the 11th post.
I hope you feel better and let us know how we can help you.