conception?

Hi, I'm new here and have a question. I have a cycle of about 28 days, measured my LH peak on Wed Dec 10 and according to the temp curve, I ovulated on Sat Dec 13. I'm now on day 26 (3 days before my period) and have had very tense, large breasts for several days. Since last night I've had light brown discharge and small cramps, it feels like the start of my period. I've never had that on day 26 before... Never had breast issues either. Could it be implantation? Can that happen so close to my period and when should I test?

Thanks, Sas