It's happened before, and I went to the doctor for it, but they said it was nothing serious and then a couple weeks later it starts bleeding again and this time it's a lot more blood. I don't know what to do!

Go back to the doctor.

I would go back to the doctor as Odinn7 recommends. If you have not had a colonoscopy, I would recommend it.

It may very well be something minor like a hemeroids, so please don't panic. I think a colonoscopy would give you peace of mind though. With that process, they insert a scope in your rectum (not as bad as you'd think - they sedate you which helps with the embarrassment factor) and they can then see if there are any polyps in your large intestine. If there are, they remove them immediately and biopsy them to make sure you don't have colon cancer. They can also see other irritations and determine proper treatment, or see if you have some other issue like irritable bowel syndrome, for example.

Did they do any tests before telling you it wasn't serious?