Could I be pregnant again?

So I had a son about 4 1/2 months ago and have been on BC since July. SInce I've been on the BC, I've been having 2 periods a month (every other week) like clockwork. Every other Tuesday. So I should've gotten it this morning or today sometime but I haven't gotten it yet. My hubby and I are a little freaked because as much as we prolly shouldn't want another one so soon, if it happened by accident, it wouldn't be so bad. Anyway, what I'm asking is could I poss be pregnant? When should I test? I don't want to buy the test and then the next day get my period...Yes, I should know all this because I've had a kid already but this situation is different.