Here, read this and be reassured!!!
copied from http://www.fetalsure.com/fetal-heart.html
Starting at week 5 the fetal heart will accelerate at a rate of 3.3 beats per day for the next month.
The fetal heart begins to beat at approximately the same rate as the mothers, which is 80 to 85 bpm. Below illustrates the approximate fetal heart rate for weeks 5 to 9, assuming a starting rate of 80
Week 5 starts at 80 and ends at 103 bpm
Week 6 starts at 103 and ends at 126 bpm
Week 7 starts at 126 and ends at 149 bpm
Week 8 starts at 149 and ends at 172 bpm
At week 9 the fetal heartbeat tends to beat within a range of 155 to 195 bpm.
The fetal heart rate will begin to decrease and generally will fall within the range of 120 to 160 bpm by week 12.
So, there you have it...it is PERFECTLY normal for a 6 week HB rate to be around 115...but of course, you should see a rise, and by 8 weeks, you'll want it to be 120+, likely it will be at least 140.....