I've been having really bad stomach problems, and it's ruining my life!?

This is a pretty long story, so I apologize in advance. Well, about 6 months ago (early January), it was the first day of our new classes, we're on a block schedule so we have 4 classes the first semester, and another 4 the next. It was already third block, and I had just finished lunch. The teacher was explaining class rules or something like that, when my stomach began hurting sooo bad. It was that feeling where you need to use the bathroom so bad your clenching up, because if you relax you know your going to have an accident. I was practically crying for about 5 minutes trying to think of something to do that wouldn't embarrass myself, when the fire drill went off. You have no idea how thankful I am for that. Anyways, I rushed up to the teacher asking to go to the nurse, and she immediately said to run. I guess I looked pretty bad. Well I went straight to the bathroom, and for about 20 minutes I was in there having really bad diarrhea. After I was done I went to the nurse, and I went home right away. For the rest of the day I was mostly fine, except the same sharp pain would come in waves but only lasted a few seconds. I figured I had just eaten something bad for lunch, so I stayed home the next day then went back to school. Everything was fine for a few days, until my stomach started hurting again in first block. I started to freak out, because there wasn't going to be a fire alarm this time. I made it through first, but when I went to use the bathroom when the bell rang I couldn't. This happened a lot. I'd think I need to use the bathroom and have bad stomach pains, especially when there wasn't a bathroom around. Basically, when I thought about my stomach hurting, it would hurt. For the whole month of February I went through a diarrhea/constipation phase. I felt like I always had gas and needed to use the bathroom. In early March we had a field trip about 12 hours away, and we had a take a bus. It was torture, I basically cried the whole time out of stomach pain. I just remember being scared I'd have to use the bathroom really bad and we wouldn't be at another rest stop for an hour or so. I went to a few doctors after this, and they couldn't find anything wrong, but I haven't had tests done. I now hate to travel, or even be in a car for over 30 minutes. I hate going to friends houses because I don't want to have to use the bathroom. I was going to go to a summer church camp, but after reading the handout on all the activities, I saw there wasn't much time to use the bathroom without a group so I'm not going anymore. I use the bathroom once or even twice a day, but I have no schedule. I never use it at the same time everyday, but I've been trying to use it everyday before school. All I think about is my stomach and my bowels! I'm thinking it's probably anxiety, but sometimes I'll have pain when I'm not thinking about it. I just don't know what to do anymore. I really need some advice.