Dull But Significant Pain Below Belly Button While Doing Push-ups.

Dull but significant pain below belly button while doing push-ups. I am currently doing p90x and just finished up the first week. Monday is push-up day and I think it may be lower abdominal muscle strain. It goes away as soon as I get out of plank position and becomes increasingly bad during the lowering and raising portion, least uncomfortable at the top.