Lie down, and if you have one, use your nebulizer/inhaler. If not try to breathe in some steam &/or take some allergy medicine. If that doesn’t help call and ask a doctor for advice.
Thanks for the advicess. I don't have breathing problem on a regular basis. So, I haven't got an inhaler. I don't even understand why it's happening. But, I'll try doing what you said
You could be sick, be near/consumed something that is causing an allergic reaction, and/or something else. Stay safe, as someone with moderate to severe asthma it will hopefully get better. If you still have difficulty breathing tomorrow call a doctor.