XXXXX,
The most common reason this is happening to you is from congestive heart failure, which is when the right side of your heart isn't pumping as effectively as it should. Congestive heart failure is typically a result of many years of high blood pressure. This is something that needs to be evaluated as it may also cause fluid build up in the lungs. Since I do not know your entire medical history I recommend you call your doctor tonight to determine if he/she is ok seeing your tomorrow or going to the ER tonight. If you cannot contact your doctor I recommend you go to the ER. Congestive heart failure is usually managed by the addition of a water pill.
http://www.americanheart.org/presenter.jhtml?identifier=4585
Dr. Pfeiffer
Doctor
Ob/Gyn Doctor