What Causes A Missed Period While On Concerta?

ok i have been taking 2 CONCERTA 36mg for about 5 years now. and at the begging of the month in april of 2010 i had a period. and i had stoped taking the concerta like mid month and i had another period at the end of april, me and my guy had sex in like the begging of may (protected) and since then i havent had a period and its the first of septemberand i have started the concerta agian but a higher dosage(1 36, and 1 54) am i pregnant???? and if so how many months???