What Else Would Cause These Symptoms ?

I am 25 years old with a 7 year old daughter. My last period was in the end of January, I have been having cramping like my cycle is going to start, but nothing. I feel these cramps at least once a week, usually once every couple days. I have also had diarrhea, heart burn, extreme fatigue and mood swings, not sure if this is related or not. I took 3 home pregnancy tests, within about a 4 day timeframe in mid-March, all negative. I am sexually active and the only birth control method being used is the withdraw method. What is the probability of pregnancy ? What else would cause these symptoms ? I am not currently, nor have I recently taken and maeications, either OTC or perscribed.