Dental implants do need several months to integrate with the bone so perhaps your implant dentist was trying to ‘load’ them too soon after surgery but it would be nice if they gave you more explanation. I think I’d ask them why there was pain, what additional work has been done to the implants and how this will help.
I’d also agree with making sure your implant dentist has lots of experience as it really does matter, particularly for multiple implants or if anything goes wrong. Don’t just pick someone who only places an occasional implant. Don’t be afraid to ask questions until you completely understand and it’s down to your dentist to make sure they explain things properly. Just to add, you do need more dental implants in your upper jaw than in your lower jaw to retain an entire arch of teeth.