(a) is unlikely and, while it might explain a slightly prolonged period of symptom, it wouldn't explain continuous symptoms ;
(b) is a possibility but secondary infections tend to attack your lungs and bronchial tracts, rather than your nasal passages ;
(c) is by far the most likely. Allergies can spring up 'out of the blue'. A friend of mine complained about his continual summer cold but I told him that he'd got hay fever. He refused to believe me, on the grounds that he was over 60 and had never had hay fever in his life. However, his doctor agreed with me. (I'm not suggesting that you've got hay fever. My point is simply that you can suddenly become allergic to something which never used to be a problem for you). The most common causes of such allergies include house dust mites (which even the cleanest houses have and are always present in bedding which isn't brand new) and chemicals used in furnishings. (It's quite common, for example, for people to start suffering apparent 'colds' when they bring a new sofa into the house).
You need to consult a doctor but, while you're waiting for an appointment, I suggest getting some non-prescription antihistamine tablets from your local pharmacy. (Don't pay 'over the odds' for the big brand names. Just ask for 'Loratadine').
Chris