My (24F) bf (24M) has been acting weird lately

so for context, we're been together for 5 years, and doing long distance for 1 year ( we met a couple of times throughout the year tho). so far, everything was good. except for the past 5 days he's been really weird and quiet. he would only respond to my texts and not really engage with me. he's usually a funny talkative guy, we would video call for hours but right now, he's really off. i asked him multiple times if there was anything wrong and he would say no, but i'm sure that there must be something because otherwise how can he explain this switch ? he's naturally introverted and sometimes he does become quiet for a day but usually it's not as bad as now. because it's been going on for days. i'm really confused because normally i would think that he's done with me, but he paid for me to come visit him in 2 weeks and he says good morning and goodnight. so it's not like he's ghosting me.

does anyone have an idea what could possibly be the issue ? or if anyone relates to this ?