Probably skin cell build up. Hairline and creases around your nose tend to be left behind. I agree pore strips for the nose work well (every third day). Here's two things that are really inexpensive you can start today - First: cleanse your face for 60 seconds (count slowly to 60). Crazy you say? NO I say! Time how long you wash right now and I bet you don't even get to 15. So lots of stuff left behind for your towell to try and wipe off. 60 seconds gives your cleanser the time it needs to get in there and clean and do some penetration into the epidermal layers.
Second: when ready to rinse, take a cheap old cotton washcloth ( you know, the nubby kind Target sells by the gross for $1.99). Saturate with water (don't wring it dry) and with mild pressure, really wipe your face down. Tip: be leaning over the sink - the washcloth will be dripping). Really get up around the hairline and nose creases as you wipe. This is a really great exfoliation technique you can use every day. Once you done a good wipe, then do some splash rinses and pat dry!
Follow with a sun screen moisturizer for day and a moisturizer with AHA or cell renewal product at night.
Seriously, the 60 second cleanse will really start to show after a few days. Now it's a habit for me. In the morning, I do a 20 second in the shower since there is no makeup or pollution to remove. 60 seconds at night! Good luck!