3rd Tone :(

Can someone help me with the third-tone rule? I've read a few books that say that a group of third tones together turns some of them into a second tone.

我很好 actually said as 2nd, 2nd, 3rd

However, in my class today I was told a 3rd only becomes 2nd if the words are connected

有小狗吗 actually said as 3rd, 2nd and 3rd. The word 小 changes because it is attached to 狗. But有 remains a 3rd because it isn't attached. Does that make sense? 

If this is the case, the I'm struggling to understand why 我 changes as I can't see how that's attached to anything. :( Any help would be much appreciated :)