Runny nose that stops and starts in an instant

I have no experience, and can only guess. ...tears. If your ducts are clear and free, not impacted, any additional tear production will drain into your nose or over your eyelids. I take it that you aren't 'tearing up' and have to dab at or wipe at your eyes, so that leaves me puzzled.
Is the discharge, when you can look at it in a tissue, any colour other than very slightly yellow? No pink stains, not greenish, not orange or deep yellow? So it's probably not sinus.
I hope you get some help here. Please be patient.

Thanks. I did read that migraines can cause this.

I am not an M.D.
I looked this up for myself because the fluid loss feels like a bloody nose when it happens to me.
There is a condition called C.F.R. Cerebrospinal Fluid Rhinorrhea. It can be a discharge of spinal fluid that occurs when there is a leak. It only affects 1 out of 100,000 people. I was not aware that it is also a symptom of migraine, which I experience too. My discharge is spontaneous, especially while bending over.

I have had 6 blood patches in my spine and there was no spinal leak. That is a symptom of a constant headache.

This is almost certain to be sinus discharge. When our head attitude changes, and with the (often/usual) rise in blood pressure from bending at the waist, especially if not just for a second, the tissues inside shift slightly and it can widen the opening in an impacted/inflamed/laden sinus, which will eject some of the fluid. This happens to me when I have a head cold. I can feel awful, have sore sinuses, bend over to tie a shoe and I'll feel a release and my nose will begin to drip if I don't catch it. I will stop the rest of the description of what happens when I blow my nose to clear it.

I have a runny nose that starts and stops without warning - and it happens a lot every day. Mine seems to be a side effect to Promacta which I take for ITP. I don't have headaches, but I have ocular migraines almost daily. No pain, but it affects my vision. I wonder if the runny nose and the increase in frequency of the ocular migraines could be connected.

I read that runny nose is a side effect of migraines. Maybe this is what you have too.

Perhaps. I've had ocular migraines for years, but recently they have increased - a lot. Maybe the Promacta is simply the tipping point to cause the runny nose issue to become so noticeable.

Everyone I know suffers from this now. Even me. It's a residual of CoVid-19 called Covid Long. Try a Netty pot with the packets of sinus cleanser. My opinion only.

I had Covid twice (once in 2021 and once in 2023); but the runny nose is new for me.