Based on those, it seems like April's charge didn't go through successfully on the regular billing date. When this happens, Spotify will send an email notification, but you won't immediately lose your subscription. Spotify will try to process the payment over the next few days. This explains why you were charged in May instead of the 19th of April.
Since you're also having issues with the payment for May, I'd recommend following these steps:
Wait for your account to revert to Free. You can check this onyour subscription page. Tip:You won’t lose any music or account settings by reverting to Free, but you will need to redownload anything you’ve made available for offline listening once you’re back on Premium.