Look dear your concern is quite natural as she is elder and has a history of COVID. You said she has a feeling of swollen tongue but there is no swelling when you checked it. Does she has same feeling in both open and closed mouth.?. There are few possibilities for your mother's concern.
1.If your mother's having artificial teeth it's may be for that.
2.If.there is any previous Restoration , fillings or any particial prosthesis it also may be for that.
3.Structures associated with tongue like nasopharygs, uvula, mucosal changes may also be associated.
4. Paresthesia of lingula nerve or any nerves that supply tongue also may do like this.
5.Brain pathology like mild stroke also may feel like this.
Next Steps
you have not mentioned the duration.From when exactly she is suffering form that.
Ask her if she had experienced it past.
If Voice changes like stammering is also present you need a multi disciplinary medical observation.
Health Tips
you observe the following conditions
1.Stamerring
2.Drooling of saliva
3.change in voice
4.deformity in food chewing
5.blowing of cheek.
I would suggest you please don't self medicate.
Go for consultation for A physician/Neurologist, an ENT, an oral surgeon.
MRI, CT scan is beneficial.
but don't worry.I hope it will recover it self.
But for confirmation just make a quick consult with the above-mentioned specialist doctors.
Thank you.