What Causes A Tender Lump On The Muscle Of Lower Back?

I have a hard lump which seems to be on my muscle on my left very lower back or butt. It has been there a month and hasn't changed. It was alittle tender at first I think but doesn't hurt at all now. I'm very healthy and just had a physical in June. I have been working very hard in my yard and working out. I don't remember injuring it but it is possible cause I'm very active. I'm not sure what to do about it