Okay, let me relieve your mind. Colon cancer in someone that is 25 years old is very, very, very rare. Add in another "very" if it doesn't run in your family. Everything else that could be wrong with your bowels is 2-20x more likely to occur than cancer. IBS is, in fact, found in something like 1 in 5 women now. Or was it 1 in 3? It is the most common bowel problem.
Most women get some sort of bowel problem in connection with their periods; that's a normal side effect of having a period, same as cramping, bloating, and mood swings. Most people won't even call that IBS, since it's obviously a period-related problem. IBS is typically something that is more or less constant and typically gets even worse when you have your period.
Some ladies on here have found that their symptoms were actually WORSE while on the pill, while some other forms of birth control can actually lessen the symptoms. If you are on the pill, talk to your doctor about trying something else. Even trying a different kind of pill might help with the bowel problems around your period (along with any other period-related problems you might have).
As for the blood and dull pain, that's from straining too hard to pass a stool. Before my gall bladder went bad, I had trouble with constipation off and on all my life, so trust me, I know what it feels like to hurt for hours after passing a really hard stool, or having a bit of blood on the toilet paper from a small tissue tear (I don't think that even qualifies as an anal fissure). I also know from having a colonoscopy that I have internal hemmroids, and those are undoubtably what tears when I have too hard or too large a bowel movement. So nothing to worry about so long as you only notice the blood after a bout of constipation, and it's a small amount, and doesn't come out when you're not having a poop, and it's very bright red.