Hi there,
I'm sorry to hear you're having
headaches, but the veins aren't likely to be the cause of them. IV's are inserted into peripheral veins, which are much too small to cause clots to travel (embolize). Usually, these veins tend to scar down when they have clots in them, and while this can cause some discomfort, it doesn't pose any major risk. Secondly, all veins in the body drain to the heart and lungs, not to the brain. Even if you did have a blood clot, it would travel to the lungs and not the brain, which is why the doctors don't think your veins are causing your headaches, although I will fault them for not explaining this to you better.
If you can give more detail about the nature of your headaches, then we can try to figure out the next best steps to help you. What causes them, how long do they last, do they effect one side of your head, any vision changes, and what makes them better are all things to note.
I hope I have answered your question. Feel free to contacf me with any further questions.