Is this normal...? TMI>

Okay so last Saturday I started doing interval running. I did 20 mins on the Saturday, 20 mins on the Monday, 15 mins on the Tuesday and 30 mins yesterday... I feel great except for one thing... I have had to go 'no. 2' at least 4 times a day since I've started and its really really loose..... I am 14 days from my next period and this morning my stomach is more sore than usual and its turned from being loose stools to diarrhoea... Is it just the exercise that's causing this or could it be a bug?