hello :) it's probably because on the first day of your period, the flow might be a bit lighter than when you fully start your period. there wasn't enough flow ( blood) to make the tampon expand all the way. hope that helps :)
Well, if this is your first period you should NOT be using tampons yet, and also on your first period you won't bleed as heavy as those who have had it for many years. Like myself, I have had mine since 3rd grade meaning that I have had mine for 7 years, so I would start by using liners or a thin pad with wings.