Reduced milk supply... exclusively feeding?

my son is going to be 4 months in few days...i have been exclusively feeding him so far..things were going smooth but from past 4-5 days i feel that there is a reduction in my milk supply( empty sooner..sometimes have to use both to fill my son's tummy..earlier only one was enough at one time) but the confusing thing is that there is no apparent change in my diet..the only thing new is that i starting using exercise bike last week. it's been 10 days so far...i am not on contraceptive pills as well..only condoms.. so i am unable to pinpoint the cause...any opinions or related experience? is there something i am doing wrong or is something wrong with me? i am also having regular periods (if that is related)..last one was in mid may.