Ode15s time step

Hi,

I am trying to solve set of odes and fit them with experimental data using ode15s solver. I faced some problems in this process. My biggest problem is shape of the resulted graph is depended on the step size as I have shown below (The graph always change at the first time step). Can anyone please explain me, is this how ode15s works or this is because some mistakes in my odes.

Thank you

Time step=0:1000: 5000000

Time step=0:100000: 5000000

Time step=0:1000000: 500000