My cycles are 28-30 days like clockwork. i had unprotected sex on day 15 and he ejaculated inside me. it is day 20 and i have been experiencing nausea, cramping, and light bleeding more than spotting. is this implantation bleeding? am i pregnant?

Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully.

You are at risk for "being pregnant" ...but your symptoms are not likely due to pregnancy. A betaHCG blood pregnancy test that can detect a pregnancy as early as 10 to 12 days after "unprotected sex on day 15" will answer your question.