It certainly could be that the elliptical machine worked the arms, too. Once your body is used to the elliptical machine and the treadmill, though, it will become cardio rather than hurting your muscles. Sometimes, the muscle pain does skip a day. You’ll do upper body weights, feel ok the next day, and then feel the pain the day after. It’s not unusual – especially if you’re lifting the weights later in the day.

I would stick with what you’re doing. Treadmill after leg weights should be fine. And, elliptical after arm weights should be fine, too. Just give yourself enough rest between weights sessions. Since your upper body is blitzed right now, a cardio day might be in order. You may not want to add back/ab pain into the mix right now. (Although, I’ve done it. It does suck, but you will get through it). The most important thing is to keep active. Do something. Otherwise, you won’t get over the initial hump. Keep up the awesome work!