28 days is an average, and changes after pregnancy/birth are normal. I'm a little extreme, but it if helps you feel less worried:
My cycle was all over the place until I had my fourth (last) child ... when I got pregnant with my first it had more or less settled down to 35 days, which was OK with me, and boy was I disappointed (but not too surprised, sigh) when it shortened up to a more 'normal' length. Now I must be at 28 because my last bunch of fertile days have been weekends about 2 weeks before my period also hitting a weekend ... with "weekend" broadly defined as 'sometime Friday to Monday' ... so I guess that tells you that even now I'm not used to the idea of "regular" ;).
(and, yes, by 'about two weeks' I mean that even that "always true" "truth" about there being 14 days after your fertile time before you get your period, could not be relied upon for me)
... when I was in college I had a housemate who was exactly 28 days and it had never occurred to her one could be anything else (because why would it? all the paperwork says This Is How It Works, right?) ... the other three of us were not 28 days (although I was the only one still irregular at that age) ... then she and the other two sync'd up (like they say women who live together often do) and everyone got a little confused for a while ;) ...
... and then a few years ago I had a housemate who was into earth-energy and such, and she looked at me like I was an alien when she found out my cycle wasn't aligned with the moon ... (I couldn't imagine having my cycle actually aligned with the moon, seriously? no wonder people think certain times in the moon's cycle make people crazy ;) ... but then I decided it *would* explain why lots of fiction books seem to speak of cycles being aligned with the dark of the moon and this type of thing ;), duh!, which *I* had never taken as anything except an overly convenient and strangely traditional literary conceit ... ) ... ironically, this housemate was on The Pill, so it's not like her body was actually having *natural* cycles from which she could legitimately rain down judgement upon my "non-natural" cycles ;) ... ah, womanhood, that amazing place of 'am I a better woman than the ones around me?' (I'm rolling my eyes here ; ) ).
If you want to track your fertile cycles (which can come at any time after your period but are in fact except in rare cases [me] 14 days before you bleed), see if you can find a Natural Family Planning trainer in your area ... I had all kinds of randomness going on that made the method a challenge for me, but for most women it works straight up for their entire fertile lives (including going through menopause, when things I gather get pretty random), and it doesn't matter how short or long or irregular your periods are. It involves either taking a vaginal temperature every day (not the method I learned, you'll need to buy a thermometer) or tracking your vaginal mucous (a gross factor here, but you wash you hands after peeing anyway, right?, and no equipment needed except plain white toilet paper). Your profile doesn't tell me if this might worry you, so I'll just toss it out there: NFP was developed for Catholics who wanted a reliable way to track their fertility (the Rhythm Method being famously not so reliable).
I have a few friends who used some kind of gadget to test their fertility, too, but I was more into the natural everyday (and no additional cost!) thing.
It sounds like you've had a really hard time :( . I hope you have a naturopath/doctor/midwife who can help you and who knows more than any of us far-away-moms do about your on-the-ground reality and can make sure your worries get answered or checked out.
God bless,
K.