Indian Passport not stamped, arriving from Canada via Amtrak

I was travelling back on the Amtrak train to New York from Montreal after a 3 day stay. On the way back, the CBP officer didn't even check my passport. Neither did he check the passport of the person sitting next to me and ahead of me.

What should I do about this if anything? I have an entry stamp in my passport for Canada, but no entry stamp for USA.

I checked my I94 on the website, and it says I entered on 4th April and exited on 27th May, but no entry for today(29th May).

Edit:

If it helps, I'm here on an F-1 visa.

Update:

I called the CBP office at JFK airport(the number for Philadelphia airport is unreachable) and they said that since my stay in Canada was just for 2 days my old I94 is still valid and that I didn't need to do anything about it.

Final update:

I just got admitted to the US again. So I guess there wasn’t a problem in the first place. I still am holding onto my tickets and documents just in case I need them in the future, but this doesn’t seem to be an issue since I just re-entered the US. Very bizarre and nerve-wracking experience, but it turned out to be alright.