I also agree with u
From few epi her expressions were diff when compared to others..
I think first ishu is suff from psycological problems...and sarika iis the first one to obseve that and may be her some hidden truth is known by shagun..so she is afraid that ishgun may split out the truth thats why she is trying her best to keep the family away from her...and thats why she may start to prove her pagal and get rid of the truth..thats why may be she changed the sindoor and poisoned ramans food so that all will blame ishgun for it...
And as she works in hospital she can know the meds that make people weak mentally and mix in ishus food as whenever raman takes food she is present there... also may incl her motive to send ishu out of balla family as she is not having that much value...
This is purely my assumption