Tell your mumsie to lighten up and that you really can't get sick from being a little underdressed. Sounds to me like you're dressing just right for the temps you're riding in. The best rule of thumb is to wear layers. I always avoid heavy jackets or anything thick because it weighs me down and can overheat me. I wear a couple of light-weight jackets when it's really cold (waterproof outerlayer and fleecie on the inside) and several shirts. Also, everyone has different sweating thresholds - I tend to be very cold-blooded so it's not uncommon for me to be wearing three or four shirts in addition to a couple of outer-layers on very cold days. Guys tend to be a little more warm-blooded and can tolerate cold better than us ladies. Just make sure that the layer closest to your skin is NOT cotton (i.e, a quick-dry synthetic shirt). That should help with the sweating.

As for your mum, well, she's just being a good mum.