After installing Garmin Connect on my new phone and logging in I lost all previous activities data

It looks like you somehow managed to Sign Up for a new account instead of Signing In into your original account. Remove the device from the current account, sign out, and then sign back in with the credentials of the original account.

I only have one email address. Does it mean that Garmin Connect would allow multiple accounts under one email address? 

I do not think so, but I saw already people having two accounts with the same mailbox, because for example they create one as [email protected], while the other one with [email protected], and similarly. And others created their initial account using facebook, and then another one using their email address. If you post you current user name or a link to your public user profile, I can perhaps tell you better what is your case.

I just synced my garmin F645M on my computer (Mac) and lost all my previous data from the same account/watch.

This is an old thread but do you know how I could restore my data on my account?

Do you still see your data in the app?

You've probably signed up for a new account. That is very common. 

No, my account email is the same and my account pic and profile information is as it was previously. I'm not sure how, but when Garmin Express logged into my Garmin account, it wiped all my previous data in that same Garmin account. My old Garmin 225 is still listed as one of my registered device, so it's not that I have signed up for a new account.

Currently I found a workaround, which is to mass export Strava data and import it manually to my Garmin account (very annoying and time consuming!). All my Gear (shoe) information will be lost, and other Garmin app-specific information that I had input on the Garmin side but this is the best way to resolve the issue.

If there is a better option, please let me know. TIA.

Contact Garmin Support by phone or chat and let them check before you do too much manual work.