5 Year Old Dog Started to Pee Inside?

I have a five year old female Bichon Frise. She has started peeing all over the house. Every time she does a pee I put her outside for ten minutes to do the toilet and she peed five times which isn't normal. She has been toilet trained, when she was a puppy. She had a urinary infection a while ago, but it has cleared up. She also had a lump on her back which lead to cancer.

She had an operation for that and has also beat it. There doesn't look like there is anything wrong with her. She is her usual self, runs about like mad and loves to play with her toys. We pay 100 percent attention to her. She loves to sit on our knees and get petted. Any solutions for what is wrong with her?

By Aimee