I'll take a dip and try..
Social constructionism is on a macro level. "We" as as society, assign labels to certain things and associate those things/objects with meanings.
An example would be money. Money is simply paper (or cotton apparently) but as a society and on a macro level, that paper can be used to trade for goods and services.
Another example could be road signs and road lines. Those signs are just blocks, shapes, and letters but they mean that as a driver, I must stop or yield or stay under the speed limit.
Symbolic interactionism is on a micro level (by George Herbert Mead and Max Weber..? I think?). We define certain things and associate those things/objects with meanings like social constructionism, but in a smaller setting.
There are 3 conditions:
1. Actions are derived from meanings. (Hey look an apple, I want to pick it up and eat it because I am hungry.)
2. We have our own meanings and these meanings can be different for others. (Apple = "yummy" for person 1 and apple = "allergic reaction, stay away" for person 2)
3. We can change our meanings. (That apple was old and icky, I am traumatized now and will be more wary of eating apples in the future.)
Symbolic interactionism deals with more social settings and interpersonal interactions.
Another example not using objects just incase you don't like apples could be a guy asking a girl to get a cup of coffee. The girl says no.
For the guy, it may mean, 'I wanted to get coffee with her and get to know her. Her saying no maybe means she's not into me.'
But for the girl it may mean, 'No, because I don't like coffee.'
I think this also deals with attribution theories here and I don't know if that's fine..but the main point is one action means one thing to one person and that same action means something different to another person.