Lumpy tummy..?

I had been spotting the whole months of Febuary and March and recently stopped in the beginning of April. I took a preg test in March and it was NEG. Last night after being intamate with hubby I realized that I felt a bump on the left side of my tummy when I put pressure on it. What do you think this is??? Do you think it could relate to digestive problems or something? I can still feel the lump. Is it just muscle?