It is quite normal for the PSI to drop over time. However, if you notice these sudden changes in less than 48 hours, then there might be a problem. For instance, if you are losing more than 1 PSI a day, then there is a good chance your tire is leaking. That is one of the signs you need to get your tire replaced as soon as possible. This also indicates that your other tires might be losing more than 1 PSI in a day.