Bruh as a person that has a very close friend to me and doesn't have much time to hang out, this is completely normal. Of course unlike Tororo, I don't have romantic feelings (or well if she has romantic feelings), but it's not pure jealousy and selfishness. It's just being sad that you don't get to hang out with your friend that much anymore.
It's like when you usually play a game together but they start playing less and less with you which makes it a bit different since you guys are more used to the play style of each other. It starts to get a bit weird to not have that person beside you in team fights since it's like telepathy when you're with them and it just feels way more comfy.
In the end, Tororo shouldn't think too much about it and just talk to Satsuki. Ask her when she's free cuz she's been missing her and want to hang out with her once in a while. It's fine to admit when you're lonely cuz your friend ain't around (or your crush idk.)