Stomach cramps

i keep getting stomach cramps after long runs. I started running in January of this year and found it very enjoyable I started off slow doing 1.5 miles and gradually have built up To 6k by march throughout building up to 6k I didn't have any problems with stomach cramps. I then carried on building mileage and decide to train for a marathon for next year, I'm following a very easy plan which requires a long run once a week on a Sunday. Ever since building mileage past 4 miles I get very bad stomach cramps after running, I always end up running to the toilet 3-4 times but it's always Normal they are not loose or anything. The cramps are similar to the cramps u get when u have diarrhea. I don't get any pain whilst running always happens after, I sometimes get wind pains whilst running but they soon pass. I recently joined a running club but I feel unable to continue going as I always get pains after long runs. I make sure I drink enough throughout the day so I'm well hydrated. I'm feeling very disheartened at the moment as I really enjoy running but actually am starting to hate going because I get pain. I'm a 22 year old female and I'm in my ideal weight range, as a young child from ages 5 year - 7 years I had IBS but i haven't been affected since, could it be this? I'm really looking for some advice please, I am considering going to the doctors but just feel silly going about something which wouldn't be happening if I wasn't to go running. I have a 1/2 marathon booked in a couple if weeks and would really like to be able to do it but currently I feel unable to. 

Any advice much appreciated