I had one in my leg (DVT) a couple of weeks after I gave birth so it was related to my pregnancy. I had warfarin for about 9 months in total - they really struggled to get my INR under control so I ended up having it for longer than I was originally told.
I think there were also a couple of dippy doctors or people working out the doses because there were days when I'd dropped significantly below what it was supposed to be and they were telling me to take very small quantities of warfarin (luckily I'd been put back on to heparin injections again but that's kind of missing the point!) and then they would be surprised it had fallen even further.
I got the tests done and it seems I don't have an underlying genetic condition, I was just unlucky and got it as a side effect of pregnancy. If I ever got pg again, I would need to inject heparin. I'd love another dc but this has put me off (especially as I'm now 43, I was 39 when it happened, i was hoping I'd get a bit more courage up after a year or so but it just never happened :( ). I also discovered near the end of my course of warfarin that if you have been pg your body reacts differently to warfarin and needs a different protocol to the normal one - again my gp and the hospital doctors hadn't heard of this, the gp was at least interested and thought it was relevant and found out more info, the hospital team were not at all interested as they thought they knew better (despite screwing up my treatment and my condition exactly mirroring the conditions described in the research).
I found I put on loads of weight whilst on warfarin - and have continued to do so ever since. I was breastfeeding (lots - a hungry baby!) and by the time I started the warfarin a couple of weeks after giving birth, I'd lost all but a couple of kilos of my baby weight, which I was really chuffed by as my weight had crept up a couple of stone after ds1 and then more whilst I was pg. (I'd only lost the ds2 baby weight but was hopeful that more would follow). But I put on weight really quickly - I've read some stuff that is anecdotal where people say that they have had weight problems with it but it doesn't seem to be an acknowledged side effect.
Conversely, ds2 is now 4 and very tiny for his age. I still wonder if this is because I was taking warfarin despite them saying that it was fine to take when breastfeeding... There's just that niggling thing in the back of my head that makes me wonder.
It was really scary - and made all the more so by the lack of information around (doubly so by the lack of information relating to dvts in relation to pregnancy despite them being one of the leading causes of maternal deaths these days).