I feel little pain in my lowest left tummy when i press it.i had been doing crunches from some days.i don't know when it started ...what should i do?

Rest: Micro tears for muscle building can cause soreness. You can do this simple test; lie down on flat surface, raise both legs straight up, then have someone press around if you have tenderness to affected area. If so, it is coming from the abdominal wall muscles. Otherwise, seek professional help.

Likely muscle strain: From doing crunches. Otc tylenol (acetaminophen) or Motrin may help. Get checked by your dr if it worsens or u see a bulge.