Could be many reasons. Don’t get yourself all worried about it. A virus of mono can cause swollen neck lymph nodes, an infected tooth, and many other things that are not what you’re reading on Google.
In fact I dealt with this exact thing with my daughter. Here lasted a long time… months, and months. They were pretty swollen too. Could see them just looking at her. In and out of doctors offices for blood tests, ultrasounds on them, her white blood count was also off which made me even more worried about the exact thing you’re reading.
They then sent her to a pediatric oncologist to monitor everything. After about we had several appointments there. She was pale for losing all her color in her skin except around her eyes were really really dark almost like left over bruising from having a black eye.
No one was giving us any definitive information and wanted to just keep monitoring things over time keeping a watch on the size and her blood work levels.
Eventually they referred her to a surgeon to remove the lymph nodes (there were two, they had been watching) they ran a biopsy after the lymph node removal and everything came back negative.
This was from about 3 years ago.
