Sometimes the head does not come fully out and the body will build an infection site to expel it. If you notice other symptoms such as problems walking, swollen joints, fever, or lack of appetite, then it is time to worry. I am going to direct you to sites on ticks so you will know what to watch for.
scroll to tick removal http://www.petplace.com/dogs/how-to-remove-and-prevent-ticks-in-your-dog/page1.aspx
Dangers of ticks. http://www.petplace.com/dogs/the-dangers-of-ticks-in-dogs/page1.aspx