Grace, be careful of trying to force anything, ever. If it's giving you resistance and feeling strained when you try to sing, it probably is being strained - which means putting wear on the vocal cords, and that will eventually cause permanent damage if it keeps happening for too long. Even a voice expert (which I'm not) can't diagnose anything from this distance, but it sounds like it's your body telling you to ease up / get help.

Off the top of my head it seems like some sort of register-break conflict, if you sing "high" for classical music and "low" (in your natural speaking-voice) for pop. It's very easy to shout and overdo things in your lower register because it feels so natural, but forcing it by singing too loud or trying to push your lower register up too far (which does sound nice and dramatic) can cause physical stress/abrasion problems, which you will aggravate the more you do it.

Do please go to a doctor - and I mean a voice specialist who knows what he/she is looking at. GPs often aren't sufficiently aware of specific voice issues. It may get better on its own, but probably only with a total rest from singing and even (as much as you can manage) speaking. Strain means you have to stop doing whatever is causing it, and let the injuries heal. But find out for sure, from a qualified expert. Otherwise you risk damaging your voice permanently, because after a certain amount of abuse they don't ever come right again.

Read some of those other threads linked in blue at the top, take a particular look at Alice's posts (you can click on her name which will bring them up), and let us know how you get on. The very best of luck to you -