what is with the headaches after the eliptical machine?

So when I started my exercise routine i was getting headaches every day. Some days it would last all day and a couple of times it lasted for a couple of days. After consulting my doctor she said to keep my heart rate at 150 or less. Well I am thinking based on my age and weight and such that I should be able to go up to 180, but I don't want the headaches.

So i have been walking on the treadmill and as long as I stay at 150 or less there is no headaches. I do have a very damaged vein in my thigh from a massive blood clot and a filter in my aorta to catch any potential clots that i may get and I started thinking that the headaches could have to do with one or both of those things, the doctor said it shouldn't. Well it has been a couple of months without headaches so i decided to do the eliptical today. As soon as my heart rate got to 120 I could feel the headache coming on so I slowed down some. Now i feel like you use your thighs much more on the eliptical than the treadmill and with that vein I really think it has something to do with the blood not circulating to my heart and head alot. I am not sure, but that is my conclusion at this point. I think that sucks because i am getting bored on the treadmill even with changing the incline. Well i don't want the headaches so I guess i cannot do the eliptical any more . Such is life I guess.