Very low if you take the pill regulary. The amount of semen does not really affect it. Usually after sex the semen will drip out, but there is always a chance.
There is a very low chance that you will conceive but I would recommend that you perform a pregnancy test in 2-3 weeks and the result will be accurate.
Yes. The Birth Control Pill is only about 96.0%-99.0% effective in preventing pregnancy. Although these percentages are high, there is always that small percentage in which pregnancy can occur, and...
No, it's a side effect, and one that's particularly common in the first three months of use. If it lasts longer than that, or is troublesome, contact your health care provider to discuss options.