you are correct that she's a little young for terrors. My guess, based on my experience with 2 children is GAS - it can be painful and startling, often jolting them out of even a deep sleep. One of the best ways to combat this is to make sure she is well-burped before being put down to sleep. Even if you have to wake her (falls asleep while eating/nursing), it is worth it to do what you can to get the air out before putting to bed for the night. Gas drops may help also - you can give "preventatively" before a night-time feeding - confirm with your doc next week. The other possibility your doctor may discuss with you is reflux but rule out gas before agreeing to putting your child on reflux meds, which are over-prescribed for infants. if gas is ruled out and reflux is still suspected, try elevating her mattress so she's sleeping on a slight incline - there are various ways to do this including blocks underneath one side of crib (must be very stable and sturdy), or a towel, rolled up like a bolster and placed under the crib mattress. make sure there are no gaps between mattress and side of crib.
try some of these ideas and do keep track of when it happens and what you tried as a remedy and be ready to discuss with your doctor next week. but, there's unlikely reason to go into the doctor any earlier. it's most likely some form of discomfort and not illness or medical problem. Best wishes to you - motherhood in the first few months is a lot of trouble-shooting and since you're most likely sleep deprived, it's best to do a little documentation about what things led to reduction in these incidents. In the meantime, simple soothing techniques (light gentle patting/burping and "bicycle legs") can relieve painful air bubbles in tummy and intestines. Important thing is to maintain a calm demeanor and darkened room when you go to her and she'll be more likely to get back to sleep quickly. She may also just be hungry, ready for the next feeding. It's amazing how that cry for nourishment can sound so frantic, terrified, and dire in the middle of the night!