Can log into outlook by not my Hotmail account

Hi! I’m struggling so so much with this, and it’s driving me mad. Last night I used a third-party app to look at my insights and stuff, but soon after discovered that such apps aren’t always trustworthy, so I logged out, and uninstalled it. I’ve always been logged into my account from my phone and iPad, but ever since last night, I get logged out of one device if I log into the other. It’s never been like this. I was always able to be logged into the account from both devices simultaneously, no problems whatsoever. After I uninstalled the Instagram followers analysis app I changed my password in fear that the app had access to my account. I’ve changed my password again since then, am able to...