Perplexed - meabt well but have offended

I am new to all this but loving my new GD of 10 months. Son and DIL often visit at weekends and all seemed well. I know they would like me to look after GD when DIL returns to work in 3 weeks. They know I am recovering from major op following brief spell of cancer, caught early. I cannot pick up darling GD and am waiting for another op. I am friends with other grandmother and visited her last week when she was looking after GD for the day. We enjoyed sharing the care. Other grandparent is going away for 2 weeks and hinted that her other daughter would be covering on those days. She also expressed concern that other daughter might need help as she has little experience of babies. When DIL returned I offered to share the care on those two days. DIL did not say very much .
I later received a call from my son asking me to arrange care with DIL or himself. He told me DIL felt that I and her mother had arranged care for GD between us and that they should be the ones to sort out care. He was clearly not happy with this. I meant well but see now that I should have been more tactful. It is all a minefield and I do not know how I managed to be so tactless. Think I will see if they contact me to ask for care on those days and leave it otherwise. I cannot offer other days as I need another person there to pick DG up when necessary. How would you play it? Any advice would be good as I feel an atmosphere is brewing and want to avoid ill-feeling. On the other hand I do not want to tread on eggshells!