Okay I can't stand it. I am a dental hygienist of thirty years. So I am getting on my soap box. First, that must be a licensed hygienist that did your cleaning. She has a minimum of two years of training before being allow to take her boards for licensure. So if that was an assistant that cleaned your teeth she is practiciong without a license. License is to be hanging in a visible are to all patients here in Texas.
Second, babies do not leech calcium for your teeth. Pregnancy does however change the quality of your saliva, increase inflammation of gum tissue and nausea or acid reflux can increase the acidity in the mouth. Also many women if nausous of tissues inflammed do not do adequate home care.
I would get a second opinion also but here are the things that contribute to dental decay. Decay is a bacteria infection in the mouth cause by streptococcus mutants (also the cause of most ear infections) This bacteria is communicable ( commonly passed from spouse to spouse or mother to baby). Over 200 medications contribute to dry mouth. Your saliva is your natural cleansing and acid neutralization. When the mouth is dry, plaque (bacterial colonies) reproduce much faster and do more harm. The plaque take sugars for the foods we eat and convert is to acid for a period of 20 minutes. This is why sipping a drink (Juice, soda, Gatorade, energy drinks) are so devistating. Take a sip, acid for 20 minutes, take another sip 15 minutes later. If it takes you an hoour to hour and half to drink a soda, your teeth are bathed in acid for that length of time plus 20 minutes. Acid pulls the calcium out of the tooth enamel,this stage is decalcification and can be reversed by using fluoride toothpaste, regular profession fluoride applications or a prescription fluoride nightly and drinking fluoridated water. If you are drinking bottled water or water run through a filter the fluoride has been removed. Many filter manufactorers will tell you that it does not remove fluoride but all independant show that they do.
Yes their are "bad" dentists just like every other profession. Detection of decay is not black or white but a judgement call which will differ from dentist to dentist. Personally with your history of lapses in your recare visits and exams, I would fill anything suspicious because a borderline area will need a root canal and crown in two years. So to avoid that, he may be recommending filling in areas that he would watch in a more regular interval patient.
Although you don't usually find that many cavities in a young adult it is not unheard of. Are any of these old fillings (more than ten years old) that are breaking down and leaking? You not only need a second opinion but a consultation with the dentist you saw and ask lots of questions. Have him show you both sets of x-rays, simple instruction is all you need to be able top read them. Have them show you pictures of these area and do an evaluation of all your risk factors. You need more information before you start treatment. And most of all you need to have confidence in your dental team.