Well, you're 16. This is the leading cause for unpredictable periods. Also, if you say your period comes every 40 days or so, there's still time for your period to show up. February was only 28 days, so you need to take that into account. Greatest advice I can give you, besides remember safe-sex, TRACK YOUR PERIODS! This will help you in a variety of areas in your life and give you peace of mind when you need it. Download the MedHelp Period tracker app on your phone. You can attach it to this account here and update it as you go. It's great!
If you used a condom, it is highly unlikely that you would get pregnant.
If your boyfriend entered you before putting on a condom there is a risk of pregnancy. A many can still have some semen come out with his fluids, even before he ejaculates.
There is no point is stressing. If you did have a mental breakdown, you would have been committed to a mental hospital.
If you do a home testing pregnancy test, you do need to follow the instructions carefully. If you don't, you will not get a correct reading.
Always write down in your diary or calendar your menstrual cycles. You will then be able to work out when your periods are due next. Don't forget we had a short February as there is only 28 days in Feb and 29 days in a leap year (this comes every 4 years).
If you were a virgin, it may the spotting may be due to the break in the hymen. Spotting may also be seen when there is an infection due to Cystitis. This is an infection in the bladder and it is very common to get this infection when having sex. Does it hurt or sting when you urinate?
If so, then you have a bladder infection and would need to make an appointment to see the doctor to get your urine examined. If this is the case, it is also very important for you to drink plenty of water - 2 litres a day is usually recommended but don't go more than 3 litres.