Is this considered being "a stalker"...?

You're not a stalker. You're just a good friend who's being hurt for no obvious reason. It looks like you've done your level best to sort this out and it hasn't worked. Why don't you phone or visit her house when she's not home and talk to her mom? Explain how concerned and upset you are because you don't know what you're supposed to have done wrong and your friend won't tell you. Tell her you're afraid of losing her friendship. I reckon you'll get it all sorted out if you try this way.