There’s definitely a kind of endorphin rush after a good lifting session. You could just be overly sensitive to it, so you notice when you come down from it more than most people.
Are you new to lifting? Are you healthy otherwise (physically, hormonally, etc.)? Are you taking any supplements?
I wouldn’t stress over it too much. Try to remember that lifting is a great thing to be doing, but it shouldn’t be “the reason you get out of bed in the morning, so if I’m not lifting today, I’m going to veg out and go blaaaaahhhhhh.”
This could be blaspheme to say around here, but… there’s more to life than lifting heavy shit. Enjoy it while you’re doing it, look forward to doing it again soon, but if you’re moping around because you’re not in the gym, we need to find you something else to be interested in.