Is it common to have trouble concentrating during pregnancy?

Could be . Pregnancy can be stressful , specially if u r very young , has limited resources and no support system.
YES it is! Especially if, as is also common, your stress level increases with problems at work, with a partner or family, with children you've already had, and the time demands and financial burden(s) of having children. Pick up some self-help books on time management and managing stress and talk to your doctor about it!