Hi, and thanks for your medical question on eHealth forum!
No, your girlfriend is not pregnant. Placing a finger with preejaculate or ejaculate inside her will not make her pregnant. The heart beats you hear is the aorta pulsing. In a thin built person, normal pulse of the abdominal aorta can be felt. This is a normal finding.
Another possibility is a strong heartbeat being conducted to the stomach area. Again it could be a normal finding or it could be due to a heart condition. However it could also be an aneurysm. This can be diagnosed and treated by a vascular specialist and an angiogram not ultrasound is done to detect it.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with a doctor who can examine your girlfriend. Hope this helps. Take care!