What Causes Rash On My Abdomen?

my husband and I are trying to conceive, we had sex on the right days and i was wondering if i was pregnant, but i did get my period yesterday pretty much on time. but i developed a rash on my abdomen and some red marks on my shoulder 8 days after ovulating. I did take pregnacy tests which were negative but I have never developed a rash like that and I had some other symptoms as well of being pregnant. just wondering if it could have been a chemical pregnancy?