Young children can't distinguish between imagined things and real things in the way we can.They struggle to explain what's going on with them.
As a parent you have to try to see things from the toddler's perspective. Fear is a normal emotion, we all have it. Young children don't know about life, so the natural fear they have tends to be about things like 'monsters'.
Having some fear is natural and helpful. People who suppress their fear have many problems. So the aim is to respond appropriately to fear, because some fears are warnings we need to heed.
The ants in her tummy could be nervous feelings, or just something she imagines is happening.
Look around the child when these things happen - when do these things happen, where do they happen, what are other people doing at the time?
When young children have fears, the advice is to talk about how you would respond, rather than dismiss the fear.
So if my toddler was scared of spiders on the ceiling I'd say 'If we see a spider, I will use the special spider pot, pop it in and put it in the garden where it lives' - I would not say 'don't worry about spiders'. You can catch an imaginary spider if it helps.
If they were scared of something in a bush I'd say 'If there was ever something in the bush, Mummy and Daddy would know what to do and would phone for help. You just need to shout for us.'
It is normal to have fears and parents just need to reassure kids that they're not alone. Fears can get worse around transition times, like going to nursery or school or a family change - that's normal disruption.