My Doggy Is In Pain While Pooping!!!!?

Well what do I do about my little doggy? This morning when I let him outside to go potty, he was winning away when he had to take a dump. I noticed that his or the rectum was extended. I put him in the tub so his was a little soft. I gave him water, and some canned dog food so his poop would be soft. He's in pain when pooping. When he's not pooping his rectum isn't sticking out. See I just spend $400 on getting my two cats fixed and I don't get paid in two weeks. I totally am broke and wanna avoid the vet. What is wrong with my little doggy?