When I lay my baby on her back she sometimes chokes on her spit could it be acid reflux?

Yes. In deference to another answer, positioning a young baby on their back for sleep has reduced the crib death rate in this country by 40%+ in the last 2 decades. Prone time is fine as long as baby is awake. Discuss positioning on an incline & other methods with your dr. Please do not put your baby on her belly to sleep for the first 6m.
Possibly. Let's not lay the baby on their back. Depending on their age babies cannot control much of anything. Pressure from a full belly is enough to "regurgitate" liquid into mouth. Clear liquid can be saliva and not acid. Choking and breathing fluid into the lungs is a risk from lying on back no matter what the source.