This is actually pretty common, particularly for people traveling to/from Canada and Mexico either by sea or by land crossings. Few, if any, of such trips result in passport stamps and often times your passport / GC are not even scanned. (You should be aware that when crossing by car, your license plate is scanned upon entry into Canada as well as entry into the US. The US also scans your plates as you drive from the US to Mexico. The government does have some way of tracking people / vehicles at land crossings).
In my own case, I had many day trips to Mexico. Some of these lasted less than 24 hours (and did not need to be reported), and some of them did. I did my best with the dates and then included a general statement indicating that I had made a few trips that lasted > 24 hours to Mexico, but for which I could not recall the dates. I estimated the number of such trips as well as the duration of the longest trip. I spoke with an attorney and she said this was fine to do, and in fact this is a common problem with people who travel to Canda and Mexico frequently. I think as long as you are not near the physical presence requirement then you should be OK, particularly if your trip only lasted a few days. Now, if your total time outside the US is 900 days or your trip was 5.8 months in duration, it may be a different story, but it sounds like you will be fine.
Remember the most important thing is to tell the truth: it's OK to say that you don't recall the exact dates and that you have no paper trail of your trip. Just be honest about it.
