((((Bev)))) sorry your hurting so much.
I've been taking Pentasa since April 2006 and it's been great for me. Of course it could be a side effect but with the vomiting too it sounds like it could be the beginning of a flare (I hope not), or when I get the flu now, it starts with pretty bad joint and/or leg pain.
If the pain gets really severe, don't just suffer through it
go to the ER.
Hope you feel better real soon.