Don't give them permission to contact your GP. They have a nerve asking you!
I once had an office job and took 2 weeks off when I broke 3 ribs. I couldn't sit down or lie down without pain but I could walk OK. Mr O was working abroad and I had 3 kids to take to school each day. My boss saw me walking them to school and kicked off, saying that if I was well enough to "wander around" I was well enough to work. When I did return to work, she was a total cow to me. I went way over her head to the top, and pointed out that my GP and the hospital said I was unfit for work, end of story.
Don't let them bully you or pressure you and ask them what THEIR medical qualifications are. Good luck.