I'm teacher, not a doctor, but I was told 'yes' by my Dr.
I was on bed rest for 3 weeks and took it very seriously and my baby is now 7. Three other ladies (teachers too) weren't so lucky. Not because they ignored advice but because they didn't get it in the first place. Perhaps the problem came after a scan. All of us were early pregnancies (1st tri-mester) and had subchorionic hematomas. We were all pregnant within 6 months of each other and friends so (over)shared details. I've sadly known of 2 more teachers who lost babies to early miscarriages for the same reason and happily, one who was prescribed bed rest and didn't. Of course, there are huge flaws in these numbers and massive biases.
The Dr told me that it is common in women who do lots of bending, stooping etc, as us teachers tend to do.
I'm not saying that I'm correct but have seen it happen and had the Dr explain it to me. You should ask doctors, not us though. You didn't ignore medical advice so did nothing wrong. If you'd been told to rest and didn't and were in this situation, I'd never have been as cold-hearted to tell you that you should have. If you'd been told to rest and did you may still be in this situation but as a teacher (HoY), you know that knowledge is power.
I think it was wrong to start giving you 'helpful advice' at this point and would never have volunteered it in real life however close we were. The fact you came here and asked means I feel like I should tell you what I know and have been told.
I hope your next BFP is easier and happier. 
I don't want my hopefully helpful post to be deleted so won't say exactly what I think about danTDM but listening to your sister-in-law over a doctor and hoping that your baby will survive makes me wonder if there should be an IQ test before being allowed to have a child.