confused on ovulation?

so some of u that i have talked to before know that i am going on 20yrs old i have regular periods. 31 days long. my last period was aug 3rd through the 9th so 7 days long, i usally ovulate on the 16th. i know that b/c i used opk's for several months and always have got a + on the 16th and i always have cramping when i O. this month we desided not to get the kit b/c we have always gottem the same results and desided to put that money towards the fertility blend vitamin we are taking. but.... i did not get my ovulation cramping untill today and so my question is why after soo long of ovulating on the same day would it change... and is there a chance i missed my peak? me and hb did bd the 11,13,15,17,18 so i beileve that if i did O before today i am covered.. but am just confused if anyone knows why...

P.s just a bit more info... i have just started working out everyday this month. i didnt know if that could change anything or not..???