What Could It Be If Bleeding, Having Cramps After Doing Ab Workout Considering Having Uterine Fibroid Surgery Previously?

Hello, I had my second uterine fibroid surgery last September. My last two periods, I ve bled in between. I thought I was pregnant so I took a home preganacy test last week which was negative, the next day my cycle was on. Cycles started on Feb. 9th and March 6th. I have light spotting again this week. I ve also been trying to lose weight so, I ve been walking the treadmill and I walk 8 flights of steps at work 3 times per weekday for that last month. Last month when exercising I was doing ab workout but I stopped because I had a bad cramping days later. Then bleeding followed the next day. Could the fibroids be growing back? could I be pregnant and the home pregnancy test not pick it up?