Mark on a calendar when you start your period and when you stop ur most likely to get pregnant in the middle of your period so like the 3rd day have sex or have sex on the 4th day it might seem nasty but it should work
Since you're so irregular,I would take ovulation tests. You can get them at the dollar store or for 89 cents at Walmart. Have sex every time they're positive.
It's very unlikely you're pregnant if you got your period so soon because it's to early to have bleeding due to pregnancy. Try getting some ovulation predictors and have sex on or around those days. Also you can't test positive until you miss a period about 4 to 6 weeks after sex. Sometimes sooner with a good test.