I agree - to have your hormone levels checked! Unfortunately, most doctors don't touch the miscarriage issue until you've had a certain number consecutively in a row (I don't know how many...)
It could most definitely be caused by low progestrone levels - a short luteal phase or the like. Talk with your doctor about this - and if you feel like you are not getting the care that you need - PLEASE find another doctor. Remember - you are paying them, not the other way around - and you have every right to get 100% service!
Let us know what you find out - and good luck!