Occasionally side effects can be observed to a product which has been used for a long time. The Frontline you bought online has the same active ingredients and concentrations as any other Frontline.
If the dog had an adverse reaction to this product please do not use it anymore. You can use K9advantix for flea and tick prevention, which is a very good product too. In my opinion K9Advantix works better on ticks then Frontline. Please read the information leaflet before using a new product.