I was on Clomid for 2 cycles (now on Femara) and found that Clomid consistently delayed my cycle 4-5 days. I would recommend getting a blood test at your Ob-Gyn (or RE's) office, as a blood test is more sensitive than over the counter pregnancy test sticks. A beta (blood) test is what confirmed for me that I was not pregnant, but rather just late. I then got my period 2 days later (4 days later than I normally would have if not taking Clomid).
I know how you feel. I am 3 weeks late after my first round of Clomid and have had cramps and nausea for 2 weeks. Home tests are negative and u go for a blood test on Monday then to my Gyno on the 9th. I have heard it can extend your cycle plus develop ovarian