You should see a doctor for quadriceps tendonitis or if you have severe or constant pain. And if you have tried a reasonable period of 6-8 weeks of physiotherapy and rehabilitation exercise with no effect. At this point, you should be managing your symptoms better and seeing some signs that you are improving. If you do not, then having a review with a consultant or sports doctor is appropriate and they can do further tests such as imaging to ensure the diagnosis is correct. If the diagnosis is correct they may be able to facilitate your progress with additional interventions such as GTN patches, shockwave or injections.
Additionally, while it is possible for the pain of tendonitis to be severe, it is possible. If your pain is extremely high and you are struggling to cope, it is important to get further investigations. This is to ensure a correct diagnosis. A doctor can prescribe you medication to help reduce the pain and facilitate you starting a rehab exercise programme.