As you say you felt no A/C movement
You might have observed the effect of you static vent becoming covered by rain water, as the rain flowed over the static vent.
On modern C150/2 C172 the static vent has round washer shape that extend 1mm above the fuselage skin, on older 172 this is not the case and the static vent is flush with the skin.
In very heavy rain it flows over the vent and stops static pressure changing. so your VSI will go to zero and your Alt will stay at the last setting that it had static pressure
This effect nearly killed me 20 years ago when descending at night IMC in heavy rain in a very old C172 and I came visual at 300 feet with Alt still showing 1700 feet that then jumped to my actual altitude.
Suggest you have your static system checked, additionally your A/C might have an alternate static source that allows cockpit static to the ALT/VSI.