I too, have very low blood pressure and had many low BP readings during my pregnancy that made me wonder. Yes the nurses, because they're always on the look out for high BP, will compliment you on it during your regular pre-natal visits. I live at 8,600 feet so I would often feel faint when I got up too quickly and when I was tired, I was really, really slow and dim-witted. Staying hydrated was super important for minimizing my symptoms and believe it or not, salt is actually good for you at this time. So if you like salt, use it liberally. Always be conscientious when you go from a sitting to standing position. As you get further along, you may feel so light-headed upon rising that you will have to grab onto something to keep from falling. My husband would often help me off the couch (late-term) and hold onto me while I acclimated. It's very scary to feel like you're going to topple when you're pregnant. With younger children, you're often playing on the floor, so devise a way to get up slowly and use it consistently. There is no med for low BP, it really is good thing but you have to manage certain activities. If you haven't been put on iron supplements, ask your doctor if they might be helpful in combating the tiredness and weakness you are experiencing. because of the altitude I live at and border-line anemia, I took iron during my pregnancies and they did help with that general sluggishness.
One last thing, I ended up in C-section w/ my last child (not because of my BP) and when the anesthesia was administered, I had a BP crash and it was really, really awful feeling for a few minutes before they took the action to elevate my BP (via IV). Make sure that if you end up on the operating table that the anesthesiologist knows you have significantly low BP, so they can be ready for that scenario.
Best wishes for a great third term and smooth delivery!