It could be a pulled muscle or even possibly a hernia, but given the your description of the location, probably a pulled muscle.
FYI, you should never be straining this hard when you take a shit. Either deal with it and wait till later or change your diet, if that's the issue. Not only can you pull a muscle, but cause a hernia, blow an ass vessel/hemorrhoids, etc.
While we're on the subject, stop wiping so hard. Wipe gently, but deliberately.
And, don't be afraid to really get in there with a toothbrush....