Light bleeding in 15 weeks pregnancy.

Hi, I am currently 15 weeks pregnant. All three pregnancy USG reports have been normal so far. The first dating USG was done in 7th week, then FTS at end of 1st trimester and third USG on 23rd January. The 3rd one was done ( along with some blood tests) because I had some digestion issues (vomiting for one day and loose motion for 3 / 4 days). My gyne wanted to check the well being of the foetus. Suddenly there was 4 to 5 drops of blood on 24th January. Then again 3 to 4 drops for 3 times on same day. I right away visited my gyne. She prescribed some medicines incl. an injection and advised total bed rest. From 25th there is no blood till now. Please advise what can cause this sudden blood. The USG scan showed positive results even day before it. Also, let me know if there is any drug-drug interactions. List of medicines below: 1) Eltroxin 25mcg 2) Sompraz 20 3) Econorm 4) Faa 20 5) Shelcal 250 6) Udiliv 300 7) Naturogest SR 300 8) Susten 300 9) Folvite 10) Dydronate 11) Proluton Dep