I agree with iwax. Strenuous activity? Overweight? Standing for long periods of time?
If they are JUST aching, it is probably muscle strain.
Try another NSAID, i.e. Aleve, rest and/or elevate the legs. If it becomes worse and/or it persists for more than 72 hours, please consult a physician.
This is one of those problems that could be minor or could be very serious.
I would be more comfortable if you had an appointment with a vascular specialist for an in-person evaluation. The evaluation generally involves a doppler image of your blood vessels. This is a non-invasive painless procedure reflects sound waves over your leg. Ocasionally an MRA is suggested.
A deep vein thrombosis (DVT), which is a possibility, could be life-threatening.
What recent activity have you done? Standing for long hours? Or have you been sitting for long hours without getting up? Could be a simple muscle ache to DVT.