If you get a bit of soreness in the Legs, it could be DOMS, (Delayed Onset Muscle Soreness). When it happens, it's usually within 36 hours of a weight session.
I often dissipates after a few weeks, when the muscles adapt to what they're having get used to, biomechanically speaking of course.
Some people always get it, but I fervently believe it's not an indicator of a good workout, but could also be an indicator of overtraining. I don't think you're doing that though.
You could try doing some Cardio on an Exercise Bike, as you should channel upper body weight into the seat or saddle and mostly away from the Legs, so when compared to upright Cardio, you shouldn't be havign to carry the added weight too.
This would be epsecially good for after Leg training, to assist recovery, because having that added resistance of the Upepr body weight on Legs, would potentially just deplete more enery stores from the Legs, and increase recovery time greater than Bike work might.
Upright Cardio is better done after doing Upper Body training, if you do Cardio after any weights sessions.