If u don't take care of you're allergies, can it get worse and turn into a cold? Ex: don't use nettipot if u have a stuffy nose, etc.

Maybe. If you are talking about hay fever, then it is a potential yes. Studies have shown that allergic people, especially those with asthma, often have an impaired immune response to bacteria and viruses. However a more common problem would be sinus infections. As for netipot for a very stuffy nose, the issue is that not enough fluid can ever get into the nose.