To piggy back off of what was already said, kids don't really express emotions just yet, and at (I'm assuming) now 4 months, a baby is only just now beginning to become interactive.
The short story is: Don't sweat it. If that's the only thing that's different, then your child is fine. As was said before, focus on the logistics of a child transitioning to daycare: Amount they ate, amount of sleep, tummy time, etc. Its an adjustment and 1 day is not statistically significant in even the most remote way.
I'm overseas also. No family close by. Two working parents. Its a tough deal sometimes. Our son was the same way in the that he didn't really see anyone else for a while. Its not necessarily a bad or a good thing at that age.
Children also mirror us a whole lot at that age. Maybe you were tired that day and didn't smile as brightly as normal so your child didn't reflect it back. Maybe your child was tired that day because he/she didn't sleep as well in a new place. I would personally be concerned if your baby was regularly socially interactive (few weeks) and then it stopped abruptly and didn't return. But that's with my child. As I said before, each day, just like each kid, is different. And adjusting takes time.
Remember: You know your child best. If something is wrong: You will know.