For the Bulgarian visa, see the comment by phoog.
Your Schengen visa has a duration of stay, a validity period, and a number of entries.
- The duration determines how many days you can stay in the Schengen area. Partial days count full.
- The validity period determines when you can stay in the Schengen zone. Unless there was a mistake, the validity should be as long or longer than the duration.
- The number of entries determines how often you can enter the Schengen area.
For a first-time application, it is typical to limit the duration to the itinerary, and to give at most a few extra days of validity to adjust the itinerary.
With a Schengen visa you can make minor changes to your itinerary, as long as you stay within the duration and validity. You may not lie about your itinerary, and you should avoid the appearance that you lied even if you make legitimate changes.
It would look bad if you enter the Czech Republic and immediately travel on to the Netherlands, because if that had been your plan all along you should have asked the Netherlands for a visa. If you spend several days in the Czech Republic it looks better.
So when you enter the Czech Republic, you should bring documentation that you have been in Bulgaria and how you will travel on and leave the Schengen area. When you travel from the Czech Republic to the Netherlands, there will be no systematic immigration checks (and no passport stamp), but there may be a random spot check. If that happens, you should have documentation that you have been in the Czech Republic and how you will leave the Schengen area.