Is my main breaker failing? Warm and weird smell coming from box.?

Yesterday I replaced the 20A breaker for just my room, as the old one kept tripping, even from just a single air conditioner and nothing else being run. Today, that breaker tripped again, with just the one 5.9 nameplate amp AC and the internet router running. Apparently, the power went off in another room in a different breaker as well, but I didn't see it trip when I checked the box. When checking the box, I noticed a weird smell and that my breaker was pretty warm. Definitely much warmer than the other breakers. I also checked the main breaker, Which is 100 amps, and that was also warmer than the other breakers. I've called my land lord to look at it, and I've turned the main breaker off to be safe, but what so you think the issue is? Faulty breaker? Faulty wiring or outlets? I should also mention that it is getting quite hot where I am in northern NJ.