I will be twelve weeks pregnant monday, here lately i have been so constipated and straining so much, is this bad?

Possibly: Straining from constipation is generally not a good idea to maintain as a usual thing, pregnant or not. I would advise increasing fiber and water intake; if this isn't sufficient, you should discuss with your ob/gyn. If your multivitamin has iron, you may be able to go to one with less iron, depending on how your blood counts look. Also, he/she may be able to recommend a safe stool softener.