aspirin is a blood thinner so it could cause bruising. Did you take a lot? A lot of anti-inflammatory medications are blood thinners as well such as Aleve (naproxen) and Ibuprofen.
Hope this helps.
I don't know if there is a correlation with aspirin and blood blisters in your mouth though. You might want to check with your doctor. Are they still there? Do they hurt?
I did take a lot of asprin over a 10 day period way more then the recommended dose to assist with the pain of shingles. In is now over a week and I have a constant blood nose as well.