Is it allergies or a sinus infection/sinusitis?

For one month I've been having lightheadedness, headaches, post nasal drip, nausea, and feelings of being fogheaded. 3 weeks back I blew my nose with a ton of yellow mucus, signs of an infection. However since then I have not seen any yellow mucus after irrigation. I feel a tiny tad better, but this is still ongoing. The big kicker for me is that for a day I can feel pretty decent, at say 80% better, but then the next day I turn very ill again. Do people who have sinus infections/sinusitis go through this as well? Or may I just have had allergies for the last month?