After waking up with 15 bites (on the second night) = sand flies bites manifest from 2 to 5 days after. Bed bugs bites manifest just after 3-6 hours after. No way you just had bed bugs in your bed only from the 2nd day.
Bites are also in a line on one leg and zig zag patterns on the other leg = typical from sand flies. Bed bugs bite on the quite totality of times on neck and hands