can period cycle change itself?

I had protected sex wth a condom with very early pull out always. I do the water test on the condoms to make sure that the condoms are not broken every single time. My last cycle was 28 days, last last cycle was 35 days, and now I'm bleeding at 21 days after sex? I'm not sure whether is implantation or just my period starting but it seems to be like menstrual blood and it's getting more.