Yes, after my emergency colectomy, I lost about 1/3 - 1/2 of the hair on my head. My hair is quite thick and the pattern of hair loss was diffuse, not in patches, so luckily it was not immediately noticeable to people who didn't know me. What was worse was when it was growing back in, as I had a fringe of shorter hair sticking straight up on the top of my head, not unlike a porcupine. LOL.
I lost my hair due to a combination of malnourishment (I had very rapid weight loss), anemia, anaesthetic, and other meds. It started falling out about 2-3 months after my surgery and grew back within 6 months of that. I also had a telltale white line across each of my fingernails, sure signs of malnourishment.
Sharon gives a great explanation, but Google "telogen effluvium" and you will find a wealth of information on what is probably happening to you.
One thing to keep in mind, if you lost your hair due to a medical illness, your hair doesn't usually start dropping until a new one pushes out the old hair, so the fact that you are losing it now may be a good sign that new, healthy hair is growing, and that your overall health is improving.
That being said - if the hair loss/thinning is not due to the above reasons, but is rather due to one particular medication you are still taking, you may wish to speak to your doctor about alternatives, or else other treatments that can augment the hair loss if there is no alternative medication.