Tobey, hello. On occasion space between upper and lower back teeth during Invisalign treatment can occur. Do not worry. It can be closed. Usually it takes several week to naturally settle. In case if it does not, space can be closed using elastics within a very short time. Usually 2-6 weeks...
Plastic will stretch over time, so it is to be expected that they distort in a case like yours. A palatal expander may be a better, more predictable option.