The marks on Lila's neck were indeed bug bites, but the real news is that Lila was six weeks pregnant when she died.
This blog post recaps the major twists from How to Get Away with Murder Season 1, Episode 7, titled HTGAWM Recap: A Pregnancy Bombshell & The Exhumation Battle How To Get Away With Murder S01E07 HUN WEB-DL X...
The look on Sam’s face confirms he is likely the father, providing a massive motive for murder. The marks on Lila's neck were indeed bug