Sex after implantation?

I am currently 13 dpo and last night we did the BD afterwards I had to pee bad and saw brown spotting on the toilet paper and a bit in the bottom of the toilet, but none since then. Could this be implantation bleeding? I was also crampy all day yesterday and a bit this morning.

Though Hcg probably isn't detectable yet, I have done 2 tests and both negative.

What are your thoughts on this? I dont ever normally cramp before my period (expected Tuesday)

Thanks in advance ladies!