Stiff spine/middle back muscles, just below shoulder blades. went swimming 2 days ago, first time in while. can i go jogging/cycling today?

Be reasonable: Your initial symptoms are a result of underutilize muscles which release lactic acid and creatine with usage. This results in the pain you're having. Once you exercise these muscles really, blood flow will increase and those symptoms won't occur. Same thing with your cycling, start slowly and then increase to prevent excessive pain and fatigue

Jogging ok: If a persons back is stiff it's mild muscle strain and should avoid cycling for a few days. Jogging should be ok, but go easy at first if you have not been jogging for a while.