You know your body well, so follow your instincts. Get the pregnancy test kit to test at home. I had irregular cycles too and I did exactly that when I missed my period (by almost a month). Turns out, I was about 4-5 weeks pregnant :)
Maybe wait 3 weeks after your last intercourse? Body changes could also be a sign, youd know your body best :)
Depending on your menses cycle.. If u think there something amiss.. Just check with the pregnancy test kit..
Depends on how irregular.