Yes low blood pressure can be caused by a few things, and some worse than others. It could be dehydration for example. It could be a sign of a kidney problem. It could be DIC if you've had a recent infection or injury or transfusion. It could be a problem with the heart. There are more so blood tests are important.
@Flycaster305, when I took shower this morning, I felt a sudden weakness in my left leg. Thanks to your warning, I stepped out and sit on the floor right away before I got blacked out and didn't get hurt. I will call my doctor's office on Monday. Thanks so much for the comment!
You absolutely should worry about low blood pressure, especially with a diastolic reading of 40 and feeling like you would faint. Put yourself in another place, what if your blood pressure dropped to such a low level in a hot tub or bath, and you fainted there.
Keep us informed.
Most doctors do not treat low blood pressure unless you are symptomatic, which you are. 85/40 is too low for most people, while some can function at those levels. The fact that feel weak and dizzy would make me want to find out what the underlying cause is for your low blood pressure. From what you describe, there is a possibility you were ill. There are some meds that can cause low BP, are you on any prescription meds?
In any case, I would work with my doctor to see what's going on.
Hope this helps,
Jon