You could be getting these symptoms for a number of reason and its difficult to answer without knowing more about your training history and more info about your symptoms.
What you must remember is that if you have been doing any form of weight training for your legs and you are not used to it, just a small amount can really affect your muscles, as they are just not used to the stress of weight training.
If you did do weights for your legs, I am not surprised that they feel heavy and crampy - the fibres are broken (albeit microscopically) and this can give you the sensation of heaviness and cramp.
If you didn’t do any weights for your legs, then there other reasosn could be:
Nutrtion could play a major part in why your legs are heavy, so address any nutrtional shortfalls you have by eating more vegetables and lean meats and see if that helps.
It is also worth making sure that you have stretch out your leg muscles well, both before and after you exercise. This will ensure that your muscle fibres are in good alignment and are able to contract efficiently.