Vegan lifestyle for about 1 month now?

I started a Vegan lifestyle about a month ago. I'm doing it to lose weight. It's been going great until just a few days ago. I got really bad stomach pains, first I thought it might be heartburn.(HB).........but I've had H.B. before and it goes away 5-6 hrs. This pain lasted over 24 hrs, much worst pain than H.B.I didn't eat for over 36 hrs...too much pain. No bowel movements for 3 days now, when I go usually 2-3 x's a day, no problem. Pain went somewhat away, so I tried a cup of black coffee and a banana. Pain came back, not as bad, but pain. Am I getting to much fiber ? If so..........how do I get rid of this pain and go again. It doesn't feel like my food is passing down.........feels like it stays in my stomach. I'm also trying to be in an alkaline state too..........cutting way back on acidic foods. Help ???? Please only answer if you are in the health field and have seen this or if this has happened to you........Thanks