If you are on your fertile window, do light cramps right after sex mean implantation?

No: Implantation when/if it occurs would be days later, not during the time of or right after the sex act. Cramps during/after intercourse are related to hat activity & the physicality of the event.

Agree with Dr. F: 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.