Yes, there's a small chance you could be pregnant. When did you see the light blood, was it the day you had sex or in the last couple of days? The reason I'm asking is because of implantation bleeding. Not all women get this but when they do it's usually within the first week after having un protected sex.
You will have to wait at least another week to take an early pregnancy test. I take it your period is not due for another 2 to 3 weeks?
I don't think it's related. There are any number of things that could cause that including something that you eat or not being hydrated enough or a lot of other things that are not related to pregnancy. I don't think in this case your issue is related to pregnancy.
It's tough to say. Yes pcos can make it harder to get pregnant, but not impossible. If you don't want to get pregnant then you should still use condoms. The pull out method is not a reliable form of birth control and shouldn't be used unless you can risk the possibility of getting pregnant. Using condoms also prevents many STDs.