For my first pregnancy, my first beta was 55 and that was at 5dp5dt, which would be the same as 3w5d. Did you do any treatment this cycle, or was it totally natural? If it's the latter, perhaps you implanted a little late.
I wouldn't be too concerned just yet. All you can really do is wait for your 2nd beta and go from there. FX that everything continues to progress favorably for you!