Hey so my car has been running perfectly fine until two nights ago. I was driving with a friend at night when I noticed that at around 5k rpm my cars idiot lights on top of my dash would all turn on at once. I didn't think much of it as we were busy in conversation however it was a dire warning for what was to come, soon after my aftermarket headunit turned off for no reason. At this point I was concerned as to why this was happening so like a real idiot I turned my car off to "reboot it" (I know how stupid this is just cut me some slack im new). Once I turned it off it wouldn't crank and I started panicking, however after finally getting the bright idea to check my voltage gauge I saw it at around 9-10 volts which was dangerously low. After getting the car jumped it started right back up however on the drive home I noticed it was slowly draining while driving. Immediately my thought went to the alternator which is the obvious solution however I also checked my terminals on my battery and saw that the negative was loose from previous electrical work. I retightened it the next morning and it started up and held at just above 12 volts. Thinking it was all good I took it on a test drive and all was well until I started to push it a little bit and soon the battery was draining again along with the damn idiot lights again. So I need help as to whether I should just spend the 100 dollars to replace the alternator or could it be something else that im missing?

tl;dr: Car draining battery when driving, started recently after late night pulls and pushing the car. Is the alternator fried?