Faint headache heaviness, small lump behind the left side of my neck, had an eye infection few days back. Could be related? Or is it related to stress

Possibly. I definitely don't think you get lumps in your neck or eye infections from stress. I think we would most usually expect a palpable/"feel able" lymph node from an eye infection to be behind the ear but the neck is not that far away! Perhaps there was some low grade infection near your eye as well. If all improving, OK to watch; if lymph node enlargement persists 3-4 weeks, see Doc. Stay well!