Well, I'm sure that CelticGirl got a bit more than she thought she would! I too, would like to see more objective data. Past and current medical history, current medications, BMI (body mass index) and even family medical history are all important considerations when trying to figure out "why can't I ...?" Another reply suggested equipment issues. What kind of bike? Proper fit, adequate gearing, appropriate tires all come to mind as additional tidbits that might help with this puzzle.
In general, I agree with the suggestion to "see your doctor". I'm compelled to qualify that by saying "seek out advice from a qualified health care professional with experience in sports medicine". An individual's PCP (primary care provider) is perhaps not the right person in this situation but may be able to refer to a specialist. Keep in mind that "a qualified health care professional" may NOT be a physician (i.e. MD or DO). This list might include Chiropractors, Nurse Practioners, Physician Assistants, Physical Therapists, Nutritionists, exercise physiologists, athletic trainers as well as others that have sport activities as a special interest.
Another thing to keep in mind is that what works for one individual may not work for another! Many of the suggestions by those replying to this post fall into this category. The suggestion to "push through the pain" or "dig deep and tough it out" could result in disastrous results.
I'm not a sports medicine specialist but I am a health care professional. I'm not a competive cyclist but I am an avid rider. I use my professional education and my cycling experience guide my riding activities.
Going back to the beginning of this reply, if I had a bit more information, I might be able to make some suggestions that could help the OP but without that information, I won't even take a guess.
Sovende