Drop in credit score

I am confused because I received an alert that my credit score dropped because of use on my credit card I have had for 3 months.  I only have a $300 limit right now, but will increase if I pay on it on time for 6 months.  I have made 2 payments so far on time, as the 3rd one is not due yet, on time & paid the balance off each time so I am current.  I just charged $70 on it, that I will pay off on time,  so if it is recommended to get a credit card & pay it on time to raise your credit score, why am I being penalized by the drop in points for using it?  I make my other debt payment (car payment) on time each time for the past 2 years (never been late), so I am totally confused.  Can anyone help me understand? Thank you for any responses.