Yeah we should all have problems like yours. The obvious answer is to just keep playing. If you keep winning you'll keep getting harder and harder opponents until eventually you'll find your level. You should know though that chess.com has an active anticheating program and the meteoric rise of an untitled player such as yourself may draw suspicion. No I didn't just accuse you of cheating; it's just something you should be aware of.
Oh and sandbagging is deliberately losing to artificially keep your rating down which you clearly aren't doing.
I'm new to online chess, due to the fact that I've always had a live opponent at my disposal.
From what I understand my brother peaked at around 2500~ blitz elo at some point during his playtime and was a tournament player. He's the one who taught me the game. I spent years (LITERALLY YEARS) playing blitz chess with my brother almost daily.
He moved out rather recently and suggested I play online chess now that he's out of the house, and wont be around to play as often.
He told me about stockfish and houdini and stuff, and I play against them daily to no avail. I am learning from them, but I missed the companionship of playing with other human beings.
I was never really a PC user, so when I tried a few blitz games on my laptop I couldn't move my trackpad fast enough to play my moves without running out of time. :(.
I have since moved to slower paced games, and with more time then usual to think out my moves I have found myself playing better all around. I am used to relying on my first instinct, or second assumption in blitz chess, so I still play somewhat quickly even when the clock is set to 10-45 minutes.
I could beat my brother around 35-40% of the time when he was still around, but I really don't know what my elo is officially. When I play with people I'm being matched with I can feel the skill difference (I'm not calling people bad, I just think I'm not where I belong.)
PLEASE CHESS.COM, WHAT DO?