Hi Em
This must be so scary for you
Aspirin reduces your bloods ability to clot. Most people say that it thins you blood and although this is not technically correct it will make it easier to understand what I think is happening.
When you are pregnant the number of small blood vessels in your vagina increases. These blood vessels are easy to damage. Because your blood is a little thinner it is a lot easier for the blood to leak through any damaged blood vessels. To help you understand this imagine a garden hose with a few small holes in it. When you let water run through this hose then it leaks out of the holes. Now if you were to let honey run through the hose it would not leak out because of how the thick the honey is. The damage to the blood vessels in your vagina can be caused by many things including physical exercise or sex. And often we have no idea what has caused the damage.
If your bleeding is only pink and on the toilet paper, and it is not there all the time then the above is what I think is happening to you.