I fell and hurt my groin. i noticed a small lump that got a little smaller its like a size of a marble. could it be a strain or pulled muscle ?

Likely a lymph node: You likely pulled a muscle and the small lump is a lymph node that came up as part of your body's response to injury. The fact that it is getting smaller is reassuring. It will likely resolve by itself. If it doesn't go away in a few weeks or starts getting bigger, you should see a doctor as it may imply an infection or even a tumor.