I have been on Pentasa for 2 years now with no side effects (that I know of).
I did however have a bout of vertigo which I don't believe was related to Penatsa. I believe it was positional vertigo. Anyways, it cleared up with some exercises. If you have vertigo, it is not a "euphoric" feeling, but more like the room is spinning, can't stand up, grab the closest thing because you're going to fall down or vomit feeling. It's actually a bit scary. I was even told not to drive while on it (I was just extra careful).
I think you're best bet is to go see your doctor and put your mind at ease. I'm not saying it's not from the Pentasa, but sounds to me like possibly something more to it than that. If it is vertigo, must be mild, otherwise, you'd know it.