What Causes Tightness And Sharp Pain In Stomach During Periods?

hi. okay so i was on my period for 3 days. the first two days were pretty heavy. the third day it was gone. && it came back on the fourth day, but it was brown and very very light. today my stomach was really tight and had sharp pains in it. my stomach felt a little harder. so i thought i had to pee. but only had a little bit come out. could i be pregnant? or what's wrong?