Do they know how long you've been in the country?

I have a question. (Sure, I'm not the first.  ) I visited my Brit boyfriend last August. My return ticket was for three weeks, but I then decided to stay for five months instead. Is there anyway that the UK will know that I stayed longer than the three weeks I was originally going for? Do they keep track of this when you're returning home? On my return home, I bought another roundtrip ticket to return to England in October. Standing in line at Continental waiting to check in, I was asked for my passport and just asked if I was going to be coming back in October? They just looked at my passport and allowed me to go in the check in line. Was not sure if they were Continental personnel or immigration.
Basically, I just want to know if they will know how long I was in the country.
Thanks for the help!

They know how long you've been in the UK each trip.  Your passport gets swiped and that information gets kept.

Does this mean I could possibly get grief when I go back next time on a visitors visa?

well you didn't overstay right? you were here for 5 months?

Yes, I was only here for 5 months. But, will they ask me why I stayed so long on my last entry? Or why I stayed for 5 months when I stated that I would only be there for three weeks?

They won't know you intended to stay for three weeks.  All they will know is that you stayed for five months.  So it shouldn't be a problem that you changed your mind during your stay.

Repetitive long stays do raise suspicions with immigration officers though.  Just keep in mind that you are allowed to be in the UK for six months within a 12 month period.  They may question why you're coming back to the UK for such a long period so soon.  Coming without a visa for purposes of tourism for six months is fine, but it is unlikely that anyone is coming over for that purpose alone if they are here more frequently than six months per year.  Chances are you won't have any problems, but you need to be prepared if they do question you.