[Alexander98]

so today at work i was sitting at the table waiting to clock into work & the girl who works there was walking over to the bathroom & she hit me on my head softly.

Could have been an accident, or a small flirtation. Who knows? I would need to see the interaction in person to form a better idea or what happened.

[mote.of.soul]

Yes, it might have been a little 'hello'. Did she smile when you looked at her?

[seesaw]

Sounds like a really immature way of flirting. Or just being a bully/jerk.

[Crazy Hitch]

Have you two had any kind of conflict in the past?

[FallDuskTrain]

Impossible to evaluate this one tiny incident, without any further context.
Lots of questions. Do you work together on same projects or are you merely co workers? Is that her way of flirting? Do you two have previous personal conflict ? Did you guys flirt in the past? Are you two good friends and she is being funny?
Is she from a different culture and this is how she shows her affection?
I mean i could go on and on with these questions.

Yes, what was the context? What sort of workplace social relationship do you have? What is the nature of her other workplace relationships? How would you describe her usual behaviour? In what manner does she usually conduct herself? Finally, how would you describe the non-verbal communication that occurs between you two? Is this behaviour repetitive?

Do you feel singled out?

There is one thing I wish to say and point out...

The usual definition of 'harassment' is that harassment is defined by the victim. It is behaviour the victim deems to be offensive, uncomfortable, etc. The opinion of the harasser and others (witnesses for example) has nothing to do with whether harassment is said to have occurred. There is another side to the definition and that stresses that the harasser is/was behaving in a way they ought to have known was not welcome.