Buoyancy is always a work in progress, I agree with what has been said before, different tanks,suits etc. have different buoyancy characteristics, different brands of bcds will be different in the way they trap air. Concentrate on breathing as evenly as possible. A lot of new divers clutch the low pressure inflator like a security blanket, I don't let open water students keep a hand on it, I preach making small adjustments with your lungs, I can only surmise your breathing changed due to lack of visibility and lack of reference.