That was a great answer jojack!
This could be ovulation spotting (some but not all women spot at ovulation and some, like me, don't always spot - to my mind (though no evidence to support this) it seems to depend on which ovary released the follicle which was ready, or you might have a slightly sensitive cervix which when sex can get passionate has been irrated slightly...but with irregular cycles it's hard to tell.
If you haven't been charting your cycles or taking your Basal Body Temperature or been taking OPK tests to check if you have been ovulating (which you may still do with irergular cycles) then you might want to think about doind so.
If charting shows irregular cycle lengths and inappropriate bleeding at times when ovulation or menstruation is not occurring then you will need to get to a doctor/gynecologist to have this checked out then - it could indicate a hormonal issue, Poly Cystic Ovarian System (PCOS), fibroids or an ovarian cyst/polyp - all of which can be treated, but some of these need to be caught early to prevent fertility issues.
The only other time you might get 'legitimate' spotting is with implantation bleeding - this is when the fertilised egg implants/burrows into the lining of the uterus to prepare for pregnancy. However not all women experience this and the same woman may experience it with one pregnancy and not another.
Hope this answer helps!