Bitewings aren't full-mouth x-rays, so it's possible you could have been developing cavities in other areas. It's also possible your pregnancy could have affected your teeth and bones. Every baby is different - a baby's blood type alone can really affect the mother in some cases. Also, the effects of pregnancies can be small but accumulate over time until they finally become noticeable.
It could also be that you're developing a calcium deficiency or something. It wouldn't hurt to see your regular doctor for blood tests.
What bothers me is that this was your first appointment in two years and the dentists didn't examine you before having you go to the hygienist for a cleaning.
I have real problem teeth and have been to several dentists in the last five years (ran into a few actively bad dentists for the first time in my life, also moved a few times). Every single one of them looked at and x-rayed my teeth before having me get them cleaned. So how is it that the dental assistant caught something that your dentist hadn't seen?
So I'm with the others who recommend that you get a second opinion. You might even consider getting it at a dental school. Tell them the story (don't say how many cavities were found) and say that if there are that many cavities, you want to save money by having them filled at the school. You don't have to actually go through with it. The point is, you'll have expert dental professors evaluate your teeth without their self- interest getting in the way. They can't teach students to fill cavities that aren't there, and it doesn't fill their pockets because they have plenty of other patients to choose from.
X-rays aren't always perfect, either. I had pain in a back molar but the dentist said nothing showed on the x-ray. It went away, and I was halfway across the country when it came back several months later. Major pain and my face swelled up on that side, so I went to a dentist, who also took an x-ray and saw nothing wrong with the tooth. He prescribed antibiotics, which helped until I used them up - and then the pain and swelling came right back. At this point, I'd left and was in yet another state. I got an appointment with an endodontist, who saw it on his x-ray but told me he used a different view or something. (I wasn't clear on this.) When he went into the tooth, it was definitely abscessed, and he did a root canal. Got back to PA, went back to my original dentist and by then I'd forgotten about the whole thing, but he immediately noticed the crown and asked about it. He was just sick when I told him the pain had come back (sick that he'd missed something that serious), and rechecked his x-ray, still saw nothing. I told him the second dentist hadn't caught it, either. Point is, this wasn't just the beginning of a cavity, it was serious enough to form an abscess - yet it didn't show up on the x-rays. Oh, and I should add that my original dentist had a brand new (just a couple of months old) office with all new equipment, so it wasn't old, outdated equipment.