Well come to HCM
Both the clinical condition and medicines taken for that as you mentioned here some name of antibiotic this may cause irregular menses, but this comes to the normal as soon as you finished the course of medicine and got alright, if you do not have period then this may not be pregnancy
, still the best way to rule out this is ultrasonography
test, get it done to rule out pregnancy, your other associated symptoms may due to stress
only, have a nice day.