I went a toilet and it was a big one (crude) and today i feel a lump on the outside of my anus and its painful! i'm scared its cancer i'm 18?

Lump: You probably have a hemorrhoid. When you strain to have a bowel movement you can cause dilation of blood vessels near the anus. They can be very painful and often itch. Make sure you drink lots of water to keep the stool soft. Use over the counter hemorrhoid cream for relief. Cancer is very remote possibility.

Unlikely to happen: One of the problems of being irregular is passing big stools that can create small tears in the skin around the anus. At your age and with this history, the most likely cause of the lump is a skin tag, growing from one of the overstreached points in your anus. Have your doc check it out duirng a rregular office visit.If you are obscessed with the cancer idea, have someone assess it at urgent care.