My tumor has swollen up?

I am a 17 year old male, i was diagnosed with Nodular Lymphocyte Predominant Hodgkin's Lymphoma in May when i was 16. I have been receiving aggressive chemotherapy for 4 months now out of 6, having just finished my 4th cycle i was sent for a PET/CT scan. I woke up the day after my scan to find the tumor under my arm which had previously gone away had swollen up substantially overnight. Is this an after effect of the scan or is this a sign that the chemo is not working?