It's most likely because you're putting to much pressure on your anus. Lifting things, moving around, just being in the heat can cause your body to tense up, thus making your sphincter close tighter. Keep it clean and put some preparation H ointment (name brand is important and it's also important you get the ointment) and put it on there every morning, after every shower, and after anytime you lay a stool. This will help lubricate it so stools can come out easier thus making the healing process begin.