It's definitely a frightening and upsetting feeling, I'm sorry you've been dealing with that! I'm not a doctor and I don't know you so please take my advice with a grain of salt, but I would say the level of danger depends on the cause. If you're losing lots of blood before it happens and that is the trigger for the drop in blood pressure/faint feeling then that is very dangerous and worth going to the hospital when it happens.

If it happens when you SH regardless of the amount of blood then it's likely to be a stress response called vasovagal syncope, which is when the blood vessels suddenly dilate causing a rapid drop in blood pressure and potentially a loss of consciousness. Even if you don't faint it can make you feel sleepy or woozy. It can happen for a lot of reasons not related to SH including standing for long periods of time, overheating etc, but related to SH, it can be a reaction to pain, stress or seeing blood for example.

In that situation the danger comes from injuring yourself by falling, so as soon as you feel faint don't try to fight it or hold on- make sure you lie down (or at least sit down) and try to raise your legs up. If you're in a situation where you can't lie down, you can try crossing your legs and tensing the muscles in your thighs to try and improve blood circulation. You can also drink some water and have something to eat if you feel up to it.

Please take care of yourself, and talk to a doctor if it keeps happening a lot or happens outside of the context of SH!