What Causes Delay In Periods And On And Off Brown Spotting?

I ve been on the nuva ring for 5 months now and always had my period on time with normal flow which last from 3 to 4 days. My last period was on May 31st to June 3rd. This month I was taking Clindamycin hcl for a week I was having sexual intercourse but was doing the pull out method there was a chance he ejaculated a little inside me but wasn t to worried. I was due to take out my nuva ring on June 25th to get my period once again I had very light brown spotting on and off during this week but never an actual period I still havent recieved my period and I m considered 5 days late could I possibly be pregnant or is this a side affect of the antibiotics.