Mary:
You know I think you are one of the most intelligent women I've met online (right up there with Scully, Leolaia, Blondie) so it pains me to say this, "You are being dumb!" The doctors had to twice cauterize an existing ulcer. 3 days of Robaxecet w/ aspirin did not cause the ulcer. Therefore you did not cause the problem!
While aspirin can reduce the blood's ability to clot (aka blood thinner) that did not cause the ulcer which was what was bleeding and which caused the problem.
Even if the aspirin contributed to the situation, the majority of people wouldn't know the difference between to two different kinds. In this case your knowledge is leading you to feel guilt and your intellect isn't kicking in to tell you that you shouldn't be beating yourself up over it.
You also took action to help your father who was in pain. How many kids aren't even willing to take out the garbage never mind do those kinds of errands for their parents.
I probably won't say this often but.. Listen to Deliliah! {{{Mary}}}
Uzzah