Hi lil welcome to HCM,
Since you have given birth to a baby 6 months before not having menstural periods is a normal phenomenon that we call it as lactational amenorrhea
The chances of pregnancy
was very less if you have breast fed your children regularly even if havent taken birth control pills but pregnancy cannot be 100% eliminated given your symptoms, So better get a pregnancy test done....
On the other hand these symptoms may be due to return of your mensturation also..