I Was Sick About A Month Ago, Got Back To

I was sick about a month ago, got back to about 90%, and now I feel I'm getting sick again. Sore throat, have had constant drainage and low energy. How do you know if you have a sinus infection? I've never had one...
I've heard if it's colored mucus, but everything is clear? Just had a drip down the back of my throat for a few weeks, and a little sinus pressure.