You say you've been riding harder and longer, lately. For longer rides, that (cheap) saddle may be showing it's true colors. I rode the same saddle for about a year. At first, it was an improvement; for a while it was fine, then, it became unbearable. I switched to a different saddle that looks like this:
Correctly positioned, I can ride all day on it, in comfort. Maybe it's time for you to get a saddle like it.