1. It could be a vanishing twin and the reason why it dropped and it may rise again
2. It's a chemical pregnancy and you will get you period once it drops closer to zero. Also, if you're still taking your meds, sometimes progesterone supplements prevent your period from coming.
According to your dates your first beta of 157 was 16 days past your 3 day transfer (16dp3dt)? The reason why they said it was low because that number is usually seen around 12dp3dt, but with beta numbers there aren't rules, just as long as the beta doubles within 48-72 hours.
Also, usually by the time of your first beta at 16 days, any medicine you took for you transfer would've been out of your system so to answer if you're pregnant, an hcg level above 5 means something happened and something attached to make that hcg level. Truthfully, for it to drop is not the best sign, but we will all hope and pray for the best for you!!