Congestion is irrelevant. This needs medical attention.
A sinus blockage may have no drainage, no signs of a cold or stuffy head...you may feel fine much of the time. Clearly something is wrong here.
Don't be fooled into thinking that what you have simply becomes a headache and then goes away again. Sinus problems can be increadibly painful and can be completely debilitating. They can incapacitate you. If you're flying with a sinus problem, especially one that's clearly manifesting itself like this, you're playing russian roulette.
I've been hospitalized several times as a result of this in the past. I had no prior indications that anything was amiss. On one occasion I went straight to the emergency room, the pain was so bad. I couldn't think, couldn't function. It felt as though I'd been hit hard square in the face with a baseball bat.
Your sinuses aren't big open spaces with big open passages...they have very small vents and openings. You may need a course of antibiotics, or other treatment. It doesn't take much of an infection or irritation to interfere with the very small openings to those sinus cavities. If you have a sinus blockage and climb to altitude, trapped air in the sinus is attempting to expand, causing pressure and pain, and it's not relieved until a descent is made. Conversely, the opposite may happen in which the sinus vents during the ascent as the air expands in the sinus cavity. When you descend the lower pressure can create unimaginable pain and cause an implosion of the surrounding tissue. I experienced this, and believe me, it HURT.
Not long ago I had a problem somewhat like yours. I had a secondary indication on the ground; when I bent down such as to tie a shoe or pick something up off the floor, I could feel pressure building in my head, and when I stood again, I felt light headed.
I visited my AME to discuss the problem. He prescribed a course of antibiotics, and a month later the problem was gone.
Remember that your medical isn't valid for flight if you have a known condition that makes you unairworthy. A sinus problem easily falls into that category. Take it seriously and get it examined before you risk hurting yourself or others.