D.B.
My son had a really vicious case of mono - he had a terrible sore throat but had not had strep. His case was so severe that it showed up on the first blood test, but FYI many cases need a second, more accurate test. So if you get negative results, don't assume that means "no mono."
The enlarged lymph node in the armpit makes sense - that's the body's immune system trying to fight this. In my son's case, his spleen was so enlarged that you could see it protruding from his upper abdomen (just below the rib cage). The doctor said it was so bad, my son should not wear a seatbelt in the car lest he rupture the spleen if we had an accident. He said not to drive except to the doctor and back, and to tell any police officers who wanted to write a ticket that it was on pediatrician's orders. So you know it was bad - pedis are always drilling parents to use seatbelts for even a drive to the end of the driveway.
My son's sore throat mostly came after the diagnosis - it was horrible swallowing and my son only ate soft foods - yogurt, applesauce, soup. In the first few days, it got worse and was bleeding. So he wasn't able to eat anything and we were close to needing IV nutrients. A health professional told us about a high nutrient supplement that was thin enough for my son to swallow (not like those thick ones with emulsifiers in them). My son did great on that and wound up going back to school in 2 days, and 2 days after that the doctor said his spleen was normal sized and there was no sign of mono. So that was a 4 day full turnaround.
My son was a bit tired before the diagnosis but not to the level that we thought there was anything wrong. He complained a bit of muscle aches, but again, it wasn't constant or alarming, just something we figured out after we "reconstructed" the time line. He just woke up one day yawning horribly, complaining of the sore throat, and with noticeable swelling in his face particularly around his upper cheeks and puffiness around the eyes.