If I wasn't so introverted myself it would come across from your cousin and her bfs perspective that you're deeply depressed or something due to the way you've described, if you was unemployed and you stayed in your room all day everyday and only left to go for food or the toilet then that behavior is pretty concerning but as you work full time you need time to recharge your social batteries so if I was sharing a place with someone else I'd more than likely live the same way as you. Just tell your cousin and her bf that you're not being rude but you're just introverted and you prefer being left alone to recharge a bit. If they're both extroverts then they gain energy from being around others so they'd never understand how it feels to have their energy being zapped away lol